Isolation: One thing I have been waiting to see again, is the separation mechanic ( Everyone's favourite; Verrüct). It made 3 and 4 man games more interesting, I would love to see that return. It could also make for great PaP/EE mechanism, willingly isolating yourself from others for a solid time (couple minutes or rounds), being able to PaP or do EE steps otherwise not possible. Also in bigger maps, every man for themselves in the beginning could be wonderful if combined with right scare factors (Imagine Origins "characters meet" intro with gameplay)
Characters: Oh yes this, I always thought hidden perks were meh, but on the other hand it would be nice to see a little difference between the characters. Small perks linked to the theme of the character: Ze Doctor reviving bit faster, Dempsey the war hero being able to carry one extra grenade/tactical, Takeo, the man of honour knifing bit faster and Nikolai, the alcoholic of the Motherland, having a bottle of vodka with him and so on. Stuff like that would give a nice touch to characters, and overall I think "Next-gen" could give great opportunities in growing the character personalities and relationships in general.
Stats system mentioned above: So much yes
Kar98k Old weapons: This is bit off-topic, but as a person who runs Armageddon on high rounds, I would love to have box with more mixed weapon arsenal. Hit in WW2 weapons, cold war shooters and the futuristic stuff alongside wonder weapons, and you're set.
Daydreaming: Modding support
--To be edited if I come up with anything else--