I have no idea with multi-player, but single player wise, I like the idea of sending it back more to its COD 4 : Modern Warfare roots with much more believable story-lines more focused on soldiering, but a shooter like COD can never really give any portrayal of combat, you want that go play the ARMA Simulations or Spec Ops: the Line for the mix of realistic engagements and the physiological experience related to conflict. That being said I would like to mix in some elements from Black Ops 2 which I actually found to be enjoyable that, its one of the only COD Games I ended up buying after trying it out with a mate. That is since another Treyarch title COD: WAW. More specially the Blops 2 elements I would like to bring over are the branching story arcs based on player decisions and optional objectives along with the ability to customize ones load out at the beginning of each mission but have scenarios change so frequently that players are forced to adapt and pick up weapons. The constant pick ups in the game available through Access Kits were also a really nice touch. In regards again to the branching story arcs, the game actually had a pretty impressive array of possibilities based on some interesting morale delima's and even player performance in the levels (Like not being able to catch up to DeFalco on board the floating city in time resulting in him being a foe later in the game and doing some pretty ghastly stuff). Though I would like to take it back a bit and break away from the shiny near future or Modern Warfare era for just one title.
For me though, one option keeps sticking out in my mind.
Call of Duty: Trailblazers or Legends or something along those lines. This would be set during the 2nd World War maybe even spanning into the 1950-53 Korean conflict. This would be based on the formation and debut operations of the first special forces set across multiple nations. This would mostly be set upon real operations. Players would take part in the original British Commando and SAS/SBS operations in Scandinavia and the Middle East, along with French Resistance/ British Special Operations Executive (I like this Idea as it would allow for a bad ass female fighting character in a CoD game as these young women were dropped to lead French Resistance groups) behind enemy lines in Europe. The Australian Special Force Z commandos/ 2nd Independent Company doing raids into Singapore Harbor (sinking Japanese warships with nothing but a kayak and some explosive charges definitely deserves a note)and fighting in the Jungles of Pacific, along with the American OSS operations in Europe or the Pacific. Heck throw in a mission as the German SS or elite parachute regiment to show a jarring comparison to the hell and carnage experienced by all forces and allow you to experience it from the other side (just as BLOPS 2 allowed you to do in some limited segments) Stealth would be massive in this game, featuring a full fledged mechanic instead of some half assed script where the enemy just has their back turned to you the whole time just so you can stab them for effect. Again branching story arcs, like say your mission in Europe as an SOE unit was to link up with the French Resistance and capture an Axis intelligence officer, the decisions you make in that mission along with your performance could effect the quality of intelligence for the Special Forces operatives in the next game. Optional objectives with less linear maps would allow for more interesting game play, whether certain objectives are marked or not is linked to the players performance in the last mission, these are still accessible with exploration but are more heavily guarded. The story would end as the Korean war starts and the cold war really begins to heat up, with the surviving special forces characters drafted into an allied initiative to create a covert intelligence force to counter the soviets.
I dunno not to shabby an Idea I hope.