Game Systems that can be used with Riddle of Flame with varying degrees of ease

Primary 

GURPS 4E

GURPS is the primary system that the setting is represented through on the blog. However, this is is a boon to other generic toolset systems because the deep and detailed system can be converted from more easily than other systems.

Secondary

Secondary denotes those systems that it is possible to use with RoF but may require a moderate amount of conversion.

Dungeons and Dragons

Riddle of Flame began first as a Dungeons and Dragons setting and a great many concepts still overlap even as its focus was made to be used with GURPS. Much of the point of the setting was to take a high fantasy setting whose status quo was shaken but things like dragon and dragonkin are very similar and there are other things that mechanically can be used as analogues like Anathema serving as the patrons of warlocks and clerics and paladins working very similarly. Really the largest issue is converting the GURPS weapons into D20 weapons along with fitting them into those concepts. I do not like how D20 works when ranged heavy settings are involved like Epoch 2 and especially 3. Along with this, one will have to fit classes to different concepts rather than having GURPS's free form system.

Savage Worlds

Savage Worlds can translate the GURPS-based mechanics on the blog into itself with a lot less difficult than other settings. With the preference of analogous weapons, armor, tools, and equipment it may often be as simple as, for example. searching for a repeater based upon a Sharps .56 in its existing form in Savage Worlds. Character stats may be trickier between the two but they share similar concepts that it should not be hard. However, the choice between the two is more based on what gameplay type you want to see. GURPS, and I the author Kadoken, prefer a grittier and detailed feel to things. Savage Worlds, however, excels at faster and pulpier gameplay with the reduction of messiness and overdetail. It is a party's choice to what they feel to follow and neither is truly better or worse than the other. I just base the blog's mechanics on GURPS because of preference alone.

The hardest thing about this one is that the playbooks are going to change via epoch and via scenario. I imagine there are a number of different PbtA settings that can be used depending on what you are using. Maxim of Steel-based games or general adventures on Shudder would work with Wild West or better yet Weird West style playbooks. Other scenarios could fit schemas that already exist.

Chaosium's Basic Roleplaying System

This system will have a harder time converting, stat-wise but I imagine there's online resources to go from GURPS's 3d6 system to Chaosium's 1d100/1d10 system.

The Secret of the Heaven Engine

Deep Lore Secret

GMs, do not freely let your players know this

The NPCs of the world do not know this.

The Heaven Engine of the insect people (the Arjit) and its ability to cause cave-ins and give its blessed powers is due to this: The heaven engine is literally a mining machine. This device that gave this civilization the means to survive against genocide and corruption is literally an ancient magic drill that just gave its makers the ability to make big tunnels, much like the Elakansi can with their totems. This thing that has a Cult of Reason-esque feel about it with the religion built and is integral to this society is literally just an aethereal drill, fundamentally. The question is, does this truly matter matter?
This will only be found out because every black spire on Shudder has one at its base, and they are one of the reasons why the lands around the towers are safer than the rest of the wilderness.

Brief Broadview History and Technological Levels of Tys

A Dearth of Knowledge

Note, the current information is based upon archaeological and anthropological findings by the time of 612 PR, the latest current start date of the setting in Epoch 3. 

The Velassian Empire, the oldest known empire, has extremely little information known about it and those that could possibly have told of it, the older Aelthi of nations such as Syldania and Cymrie, have both died and oddly had no memory of the empire when they were alive. Along with this, no records of a long-reaching empire have been kept. Even the younger kind of Duerthi seemingly have no records nor memory of an empire that their old kingdoms existed alongside. Divination, indeed, seems to have been blocked off from scrying before that date. One explanation for this may be a great aeth-based disruption in the skines of fate that was purely coincidental. Seemingly supporting this theory, this was the time that great geographical changes occurred with the Great Astral Rain of the -20,000s that formed the unnatural Fangs of Skalovia, the beginnings of the archipelagos of Grush and Liresku, and the smatterings of other star stuff that fell all across the planet. This was also the beginning of the murmurs of the Worms, that ancient enemy of the kinds. These along with other events may have caused a sort of tear or essentially created a magic-based static for this time. A second theory is possibly great willful sabotage of the skines, destruction of records, and enormous magical tinkering of memory itself. This theory is supported by the specific absence of old Velassian records of any manner along with the absence of memories by the old kinds.  Along with this, the relatively recent arrival of the Drathi and their strange snow-white skin and use of previously unknown blood magic points toward this second theory. The Drathi too have a strange lack of memory of where they came from, just that the old Velassian empire was coming back and were dangerous. It has been theorized by the miniscule few believe them that the Drathi are possible outcasts from a greater unknown empire built by the Velassians that fled long ago.

However, the Velassian Empire is not the only forgotten kind, for the predecessors of the green-skinned urkain have also been forgotten. That forgotten kind actually does have images of them built upon archaeological findings and may have uttered the Twin Gods Dhaegoth and Vhaegoth into existence.

One more lost record is that of the Leviathans, the precursors of the dragons.

?-(-20,000 ON) Forgotten Twilight (GURPS TL ???????)
The technology level of this time is unknown. However, very old artifacts, seemingly the only remaining things of the ancient Velassians, may point to this time as being extremely prolific in magic. Magical orbs, wands, staffs, wards, and equipment that are fully made of and run on magic were found in the world and assumed to be from this time. Older tribes and smaller kingdoms may have been based on ownership of these. These artifacts were also the starting point of the development of aeth use.
The beginning of the Gromdalian calendar begins at year 0 during this time. The calendar used is the Lothrian calendar.

(-20,000 ON)-(-14,000) Epoch of Bronze GURPS TL 1
Dwarves and Elves close off their borders, either digging deep into their mountain lairs or hiding within their magical forests. The kinds are left with essentially the detritus of the old Velassian empire.

(-14,000)-(-8,000)Epoch of Iron GURPS TL2+1

The dwarves and elves venture from their closed homes and begin trade with humans. Magic and divine power are able to substitute for advancements.

(-8,000 ON)-(0 ON) Epoch of Steel GURPS TL3+1
Feudal kingdoms will be prominent for most of the era and then transition to Absolute kingdoms and the beginnings of Harmonism, Materielism, and Equitianism show up during this time. The Pentaphany, the pantheon of the five gods (Sothe, Liestra, Alar, Prythe, and Astres), gains prominence and spreads across most of Velas and the southern half of Xatla except for Zeth. Drathi is established in -1532 ON. Magic and divine power are able to substitute for advancements still. 

Armor and non-gunpowder weapons analogous to those existing in 1500s on earth exist.

Gunpowder is in its extremely early stages and the use of magic and divine power along with proto-magitek in the form of runes causes a stagnation in its research and use. Until Uhlion the Broken discovers its secrets via unknown means and uses a changing team of researchers and inventors to bring it to pragmatic use by -40 PR. These secrets mysteriously spread to other nations but investment and adoption will not occur for decades until the Skalovian revolution shows its power. However, even then those armies armed with it are supported by old standbys like clerics, paladins, and mages along with so it is still considered to the majority as a niche tool that worked for the Skalovians for decades by a majority of peoples on Tys.

Essentially, the age of technology jumps to 1815

(0 ON - Now PR) Epoch of Flame

Epoch 1 TL5

Epoch 2 TL6

Epoch 3 TL7

The Secret of the Modern Kinds

GMs do not show your players this

This is an answer to a question that will not be answered until the 3rd epoch in the 6th century of the world.

Almost all kinds are the result of lab, magical, or unknown experiments performed by aelthi, dragons, or other secretive precursors. Aside from the insect people known as the Arjit and their related kinds and the aelthi, drathi, duerthi, and dragons the known lands of Central Tys are inhabited by essentially purposefully devolved versions of their precursors. Living aelthi, drathi, duerthi, and dragons do not remember this and it is unknown why that is.

This is why they look similar and are universally bipedal, why classes of kinds can intermix, why they're able to speak similarly to each other, and why animals they resemble are still around. The ones with this knowledge left are the old Velassian Aelthi. In the third epoch, they will return to break up the cold war between the Uhlionic Republic and the Thralish Commonwealth with their invasion and reveal this to the world as a whole.

However, while it's most likely true that urkain, uglotzi, and yagotzi are most likely similar experiments, their precursors are unknown. Only perhaps the Twin Gods of the Urks know, with ancient archaeology on the Grush Archipelago depicting strange ancient figures using similar symbols...