Remember the X1's drive caps out at about 340gb so it can fill up quickly.
But yeah, just delete the games when you're done. It's not like the installation times take forever anyway, if you decide to play a game again.
It tops out at 362GB actually. So you can have up to 23 of the 27 currently available games (depending on which ones you have installed) on the XBOne at one time if you don't have any other apps, videos, or other content stored on it.
TC - As has been pointed out, just delete the games when you return the rental. You don't lose the save progress because that is in the cloud.
In regards to your actual question, I'd say somewhere in the neighborhood of E3 or sooner. I don't see them sneaking it in without a bunch of hullabaloo. --- You can put icing on a turd, but that doesn't make it a cupcake!