Aren't Doomsday and Secret Wars already a two part story? I haven't heard anything about rumors of them doing a third one, so I'm not sure where that idea would have even come from.
They're already doing a Shang-Chi team up movie, it's called Avengers: Doomsday, and there's also possibly even going to be a second one called Avengers: Secret Wars.
He will be in but I was thinking more about a movie were he is the focus or shares the focus with secondary character. You know who would make for a interesting pair. Shang-Chi and Doctor Strange. That or they might be wanting to build up their X-Men characters so Shang-Chi and Jean Grey.

Heck they keep saying they still want to use Blade. That would make for a interesting match. Also I hope they go for a less mystical setting and go for something more grounded or just something you would not expect. I am waiting for the first MCU movie to mostly be set in Canada! I can already see the big 3rd act fight happening in the Rogers Dome during the middle of a Blue Jays game. Watch the fans and players running to get out of the way as two power weights use magic and whatever the main bad guy uses against each other.