I'm not a terrible fan of most FPS games. Most sorta bore me with its repetitive game play.
Gimmicky guns detract from story telling as it tends to make the guns the main point of the game. Gimmicky mechanics has a similar issue, like I think fracture?? The one where you could adjust ground levels.
It would be really nice to have an organic feel for the weaponry for the universe, that exist because they were needed, and not because they are cool. Red Faction does this mostly (haven't played after red faction one, stopped once I got bored with burrowing through everything.), as does Halo, and COD, MOH (though thats a side affect of its genre mimicking RL weaponry, except a few fantastical games in the setting.)
Something that also breaks my immersion from FPS is seeing the rails of the environment that forcing me to go to Object A, especially if they are very overt like invincible chain link fences, or invisible walls. GOW2 has painfully visable rails, that detracted me from the game play as if felt like I was on an amusement park ride. MOH: Rising Sun does a wondrous job of hiding its rails through the game, (About 60 percent the time.) Halo series does a much better job at through out. TF2 has an interesting mechanic for this by making the unplayable areas of the game look uninteresting, and thusly not encouraging exploration. (If you could go over to them, which you can't.)
Covering mechanics mostly suck in games that attempt them, they make you feel glued to a surface, on tracks instead of being area and action context sensitive. I don't have any real suggestion on how to make this better, just that contemporary tries at this mechanic have been unsatisfactory. Though it did seem that KOTOR series (not strictly a shooter game), did seem to use a cover system that didnt make you glued to the wall, as did Dues Ex, but that may have been due to it its RPG elements.
Guns, all guns have a spread of shoot making concussive pin point accuracy nearly impossible and a true feet when accomplished in reality, this is generally mimic in games with a widing targeting reticle with extended firing times, this works fine though a poor replication of what its trying to simulator. There is a metric for this spread, that escapes me, I'll edit the post if I can find it again or remember. It would be nice to see a different approach to this affect of guns.
There are various antics with guns though small, would be nice; holstering, brandishing, switching firing arm, safeties, firing modes, willful steading of aim (holding one breath, firing on exhale, firing between heart beats).
It would neat to have antics in relation to these antics.
If this is going to be near future, there are various techs that will be available soonish, as as the landmaster system that been in dev. by the army for almost 15 years, sonic weaponry and the coolest I think is power armor.
Power armor of any weight though shouldn't slow down the infantry then what the expected movement they are suppose to have now, as that one of their major advantage. That is if you going for a realistic approach, if its for a hard balancing mechanic, then go for it.
There a few things that exist right now, that are very awesome, as as UltraClot, a room temp clotting material that anti septic, and can be self applied with ease that doesn't require opening the wood for suturing or covering.
There are various armors being looked out currently, into ceramics, nano carbon tubes and few other freakish ones, like one looking into replacing a certain fish scale for its kinetic impact resistance and low fracturing.
Probably the major step for hand held weapon would be caseless ammunition and reducing its moving parts, and move toward being made out of polymers instead of alloys for reduced weight and higher readability.
Direct Energy Weapons being man portable is probably out of range for a very long time, and would have to be a far future +60 years, though machine mounted versions and artilary could be around before then.
If power armor is in play, then its reasonable that a break through in researching room temp or near room temp superconductors has been made, in reach case linear motors, gauss weapons and rail guns would be probably be more likely, as they have less moving parts then a DEW would, be more reliable and easier to repair. They also have the great advantage of not requiring additional volitive chemicals with them, that lasers, ion, lighting guns would, and just be using dense inert farris(sp) pellets.
Things like anti matter, would probably never be weaponized, due to its extreme expensive in making it, justing making a few pounds from estimates for anti matter power space craft, would bankrupt the world econ and that something would require a far future setting for it to be plausible.