Offline stream play would be great.

On the streaming aspect, here is an idea. Go into your Xbox settings and choose 'Go offline'. You'll obviously have to choose a game that you can play offline to test it, but after you do that, see if the device you want to stream to can find your Xbox and
connect to it and play.

If you can do this, there should be no issue with data usage at all. Although, TBH I still don't think you'd be using any more data through streaming than you would through normal gaming on the console. The streaming should be happening on your network,
not through the internet connection. The only reason you'd 'need' to be connected to XBL in order to stream would be if it were a game that required internet.

If you can stream with your Xbox in offline mode, then it should work the same as it does if you're on the console. You just wouldn't be able to get your achievements and whatnot until you go online again. Anyways, just a thought.

Forza Horizon 4 has some great online features but it can also be played completely offline as well. Probably not the style of game you're looking for but thought I would throw it out there. Far Cry 5 is a great offline game too.

The xbox 360 was a great system for streaming from my PC. Why would you abandon something that worked so well. At least my Play station still streams from my PC

And when the Cloud goes down which it inevitably will from time to time like all streaming services do, then what....can't play streaming offline.

I think that would be a great idea. A stream or rent service would be great to have as well.