Take a look at Marvin Martin. He should be 24-25 in 2016. He is class, he can set up goals and dictate the middle of the park, heralded as the next Zidane. At the start of the game, he will set you back between £20-35 mil, but he is worth it, not least because you can get easily 10 years of solid top-tier performances from him. And with a team like City, I doubt his price tag is any cause for concern.
If you can't sign him, take a look at Kwandwo Asamoah and Sandro. Both are young and can contribute both defensively and offensively. They are well rounded and will contribute to all phases of play. They shouldn't create to big a hole in your wallet, but then again, you are City. If worst comes to worst, you can always use monthly installments, which will only add to City's grossly huge debt.
Hamsik is another player that would add to any line-up. Having said that, his age in 2016 (should be about 29) might not warrant the kind of money you undoubtedly would have to cough up. A cheaper, and younger, alternative would be Ivan Perisic. Both are fantastic midfielders with the finishing stats of a world class striker (17). Both will make the opponents net bulge on a regular basis. The difference comes in their price and age. Perisic, with a market value of £3.7 mil at the start of the game, will cost you considerably less, and is a couple of years younger.
All the players I have mentioned are world class and could easily break into the starting XI of any side, including City's.
Also money isn't an issue as Milner will be getting sold..