The real life LAPD issues Glocks as standard (I believe graduates are issued either the 22 or 17) so safe money would be on a Glock being featured as it is a ubiquitous police gun (and cops get killed a lot in GTA). Deagle is also a safe bet (as stated above, everyone loves a Deagle) but I'd like to see the Auto-Mag from TBOGT return as IMO it is cooler than a Deagle. A revolver should be featured (country cop/sheriff could carry a revolver and I'd assume gangs would carry them as they're cheaper [again, I assume]) and I'd like to see the Ruger GP100, but I love revolvers, so will be happy with anything.
SMG's; Uzi/Micro Uzi, MP5 (or derivative) and something like a Tec-9, would like to see the FN P-90 or MP7.
As for shotguns, I predict the usual sawn off (either straight SxS sawn off or a "cruiser" model pump), a police issue semi auto (again, I believe the LAPD issue/use the Benelli M4 Super 90) and then something with the "cool" factor like the AA-12, Pancor Jackhammer or Saiga 12.
Rifles: too many I'd love to see, but predicting AK47/AK74, M4 (again, LAPD issue)
Sniper Rifles: usual suspects, Remington 700 (as featured in GTAIII, VC, SA and GTAIV) and the H&K PSG1, would like to see the DSR return as well as something like a Chey-Tac or AWM.
I'd expect to see the RPG-7 return, but maybe Rock* will mix it up with something like a LAW-80 or M20A2 FLASH, also maybe some form of anti-air launcher such as the Stinger or Starstreak system.
Sorry for the few wishes in the list, but these are what I'd predict
Also, as an afterthought, whilst the LAPD issue Glocks, I believe officers can choose to carry from an approved list, and obviously gangs carry a variety of pistols and SMG's, so there's a lot of potential.