While increased application of such weird and wacky designs would be awesome considering that weapon types in games have pretty much been fast shooty SMG, superpowered slow firing sniper rifle, medium range assault rifle, I mainly just want Sci-Fi guns that seem like actual guns, that don't look like they're entirely made out of cheap plastic, that sound like lethal weapons, that actually seem like the soldiers using them wouldn't laugh their asses off upon being issued with them. A Sci-Fi weapon doesn't always have to be all shiny and goofy and crap, and for me at least it can make the entire setting feel kind of stupid if its meant to be plausible, mature and grim and has stuff like that all over the place.
Killzone is probably the best example of guns I can think of which both feel slightly futuristic while still feeling like guns, however this is probably because 90% of them are 'inspired by' (ie, ripped off) real world designs.