Xcloud games extension?

Will it be possible in the foreseeable future (beginning of 2020) to play all games available for Xbox on the Xcloud streaming service?

So should I buy Assassin's Creed: Valhalla someday, can I play it on my tablet via Xcloud?

I do not think so.

The games are either streamed from the console or online from the gamepass.

As long as the game is not part of the gamepass, you can't stream it.