NO , you want part installation that not possible

You can activate them and not install them or play them , but that is not what you are asking.

look even if a game devs do this, it still need to fetch data.

example Blizzard use a you can play now, while it still DL data , but most will say not worth it,
if you need DL zone in where you are it lag like crap.

and play hardcore games and you die, thats your problem. ( so NO NO NO never going to happend is alot better to say. ) no gamers today will accpet laggy games ever.

ps.
dont come here with start DL and the pause them, even that dont make any sense, as you see alot of yes you can and yet have some loose files , and game will not start make no sense at all.

Thankyou for your reply. The thing is, this would be easy to resolve, since for ex. the Ubisoft Activation AppID is already accessible through SteamDB and can be run. So having Steam and Ubisoft Client run would allow activation on Ubisoft Client without the full installation of the game.