I'd love to see some proper weapons or reskins for starters, normal weapons that is. I'm not saying the special ones suck, it's just that... damn it I want my guns!
I'd kill for (and with) a proper GDHC and a good ol' fashion minigun and AK47-esque weapon. A Scarface style M16 with attached grenade launcher would be great too.
SR IV is also seriously lacking proper super-cars. Would be nice to have a DLC pack containing cars based off Koenigseggs, Porsches, Ferrari's etc...
That's just some stuff I'd like for my personal taste though, what I really think SR IV needs are three things:
1. A "Classic mode". Or better yet, more options for customizing the Steelport simulation on the computer aboard the Nabuco-Normandy, that allow you to return the city to normal (law enforcement and army instead of aliens, glass towers instead of alien towers, dynamic weather, etc) or disable super-powers for yourself and homies. It would be cool if you could switch between super-power alien slaying action and old-school law enforcement rampage at will, or combine the two options and kill aliens with nothing but guns, or crippling the army with your super-powers.
2. More activities. Both new and old-school. Like maybe add some cyber-esque trials similar to the mission where... (spoiler, highlight to read)
>> ...you rescue Matt Miller from his simulation. <<
Or bring back some old ones like Trail Blazing, Septic Avenger, etc...
3. Basic diversions (singleplayer, co-op or pvp in co-op like Cat and Mouse) that give you something to do when you're tired of doing the same activities over and over. Like racing (VEHICLE RACING!!!) or death-matches (WITH GUNS).