Blood decals on the walls sounds good.
More particles might cause problems for those who are running trem on low fps on beforehand.
About the gibs, i dont think so. Do you thing a tyrant would be that badly mangled by pulse or machinegun fire, or even luci fire? they are pretty tightly armored with all that carparace.
as for dretches, dunno, they look nice, dead as they are.
but there could ofcourse be some variation in corpse graphics...
for example, the humans should be abe to fall one way or another (if they don't already do so) maybe a bit dismembered if they were, say swiped by a tyrant, but not bitten by a dretch. so, the aliens (smaller ones) should be abit messed up after walking into a fully- charged luci, or a nade.)
of course none of these are necessary....
but it would be fun.