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.