The plan for WrestleMania right now is to have The Miz defend the Intercontinental Championship against Braun Strowman, according to Dave Meltzer in the Wrestling Observer Newsletter. But as always, plans can change.
The reason why WWE had The Miz defeat Roman Reigns relatively cleanly on Raw is because they want to give Miz a push and want to make the Intercontinental Championship match at WrestleMania seem bigger.
There are apparently some people in WWE who want Braun Strowman to win the Universal Championship at the Royal Rumble, but as of now, that’s not Vince McMahon’s plan and the rumored plan for WrestleMania is still Roman Reigns vs. Brock Lesnar, with Reigns probably winning the title. It seems like WWE has been building up to that for months, so it feels like it would not really make sense to put the title on Braun Strowman right now.
Brock Lesnar’s WWE contract expires after WrestleMania and has other non-compete clauses through August. It’s still unknown what will happen and it is possible that Lesnar re-signs with WWE, of course.