I would look forward to see some levels have variety of structure and their purpose, like for example capturing enemy's base or a turf, capture the flag levels, domination levels, defending some places against the enemy ( similar to zombie waves maps except that they're armed ).
I also would like to see some new characters, like new allies of rogue/prisoner CS, Usurps or falkoks.
As well as introducing new objects, like a functioning mines or a health supplier ( similar to that of engineer's thing in TF2).
And the atmosphere and the environment, and that will attract me the most;
Make the levels have some sort of post alien invasion vibes, destroyed buildings, debris, destroyed vehicles, rusted stuff and etc.
And would be amazing if you added some sort of environmental factors that damage you besides acid, like radiation from CS BNG ( if it bullets contain nuclear atoms ) or just anything.
Fogs, heavy clouds, dark clouds, anything that attract the eye would be cool
Lastly, make the campaign accurate and realistic, in a way which it continues the storyline of both 2 and FTTP.
I'm sorry if my points are kind of nonsensical or doesn't make sense, I just did this in a hurry.