This is my opinion on what Black Ops 3 needs. (Whatever Black Ops they will call it)
Treyarch does the best creating maps IMO. Keep the 3-lane maps that you guys always make they're better than the ones with a bunch of crap everywhere. Keep the smaller maps. IW really messed up with maps like Stonehaven and Siege, make more maps the size of Raid, Standoff, Studio, etc.
Create a Class:
Keep the pick 10 system or make it even better. That is hands down the best CaC system.
League Play was great. Bring it back.
BRING BACK SCORESTREAKS! Please add a MOAB or KEM type of killstreak. That bummed people not having one in BO2. Maybe add a type of specialist package. Bring back the RC/XD and the paths for it you had in the original Black Ops.
Keep 2 round domination. Bring back Hardpoint. Add more party modes. You guys need more modes like Infected and those type.
The colors for Treyarch games are always better than IW so keep them. More camos, more vibrant or animated like Cyborg. Gold and Diamond or other gems. Reticles and their colors, lens colors, maybe even iron sites colors. Colored clan tag options. Maybe even camos for equipment, grenades, tomahawks, claymores, etc. Soldier customization.
Keep up the good work.
Try and make it the best connection yet. Destructible environments. Dolphin diving.
Though I won't buy it without seeing how it plays first(No...I did not buy Ghosts), I would change the following:
New and varying in size maps. Don't make them all small or huge.
Toughness should not exist in any form for any weapon type. Neither should any perk that allows sound whoring. For that matter, take out stock and quickdraw in all forms as well (see weapons).
C4 should be planted, not thrown.
Make classes of weapons actually matter. Don't up RPM on guns that require more bullets to kill just to make every weapon almost the same. There should be pros and cons to each weapon class and variance among each weapon in that class. Liked the idea of Marksman class from Ghosts but not sure on implementation.
Dedicated Servers only. There are too many reasons they are better than the manipulation, inconsistency, and general crap that is the last system of host based/peer/listen/whatever system they used. I won't go into lag comp because I know the reasons to have it and dedicated servers would address a lot of the problems they had with it.
The usual. Better placement based on location of enemies and more possible randomness. It could even kick in at different levels based on location of enemies.
Fix where bullets/tracers/etc are fired from to prevent head glitching while keeping cover.
Just a few off the top of my head.