You know that re-making Doom 1 engine in stencyl would be entirely possible? Alexin worked on it for a while, maybe he made progress (first demo displayed just plane with doom wall texture which you could rotate). So FPS is possible (but, of course, hard). And with adding freelook we'd have Duke3d clone (the only difference in duke3d and doom graphics engines was that in D3D you could make room under room and in doom you couldn't)