If you are looking for non-linear gameplay, try Deus Ex : Invisible War. It is an RPG, FPS mix but the fighting system is FPS...
<{POST_SNAPBACK}>
Or then there is Morrowind.....big yes.....linear yes, Every damn tomb looked the same, every damn ruin looked the same, every damn cave looked the same, excellent to start with, then becomes boring. Variety is the spice of life! KotOR kept the variety going....sort of.