For me personally, i think they need to improve the snipers, on BF4 the scout elite was terrible yet on this its pretty good? the next sniper AWH i think its called? that had some really terrible hit markers, even if you get head shots with it you're not always guaranteed a 1 hit kill.
For me not being able to have straight pull as a third attachment kinda ruins the fun of sniping as there is no Bipods and if you wanted to take say the laser sight, you can't if you also want the straight pull. I get that snipers aren't as useful in the majority of maps but that's all i enjoy doing on battlefield.
Most snipers need better range, i find myself being outgunned by sub machine guns over long distances for some unknown reason, and assault rifles seem to have better range than snipers, they seem to have nerfed snipers big time in Hardline.
Anyway enough of my sniper butthurt onto the next thing;
I think also there needs to be a bit more of a selection of weapons, Secondaries should be available to all classes, what i mean by this is that pistols specifically should be an available sidearm choice for any class along with those class specific like machine pistols for the sniper class (not sure why they have done that).
I also believe they need to re balance the start weapons, i seem to find the starting weapons for each class 9/10 times are better than any you unlock afterwards. the MP5K i personally feel is miles better than any of the others, as with the first shotgun, much more deadlier than the rest.
My main disappointment is the lack of vehicles for conquest large, and how easily the vehicles get destroyed, you barely last 5 seconds in a heli as 9/10 times it takes a single stinger to bring it down (at least whenever i'm in one as it always kills me).
The game is really fun however i'm a little disappointed with it so far as the multiplayer for me isn't as good as previous battlefield games. For me BF4 is still the best.