When I say “RPG material,” I mean things like statblocks, classes, etc.
Savage world Rifts. The setting is fantastic, the 1st party rules are a mess as the best of times. Savage worlds rules set enrich the rifts world by making it a playable.
Honestly, especially after their team up with Paizo, SWADE has been really, really improving and become my favorite system to reskin things in.
Hell I’m thinking about running a Curse of Strahd campaign in Savage Worlds.
In the same vein, I was in a Shadowrun campaign that switched over to the Sprawlrunners rules for Savage Worlds (which can be used for either Shadowrun or Cyberpunk). Savage Worlds brought out the dynamic action and calculated hacking scenes that define good cyberpunk fiction without being dragged down by clunky rules.
It is an unfortunate fact that Shadowrun tends to suffocate under its own density.