It's been reported that the original plan was indeed for Big E to retain the WWE Championship at Day 1. Brock Lesnar was inserted into the match to make it a Fatal 5-Way, and ultimately walked away with the title after putting The New Day member down for the 1-2-3 thanks to a brutal F5.
On Wrestling Observer Live, it's been confirmed that Big E was indeed supposed to retain the title before Roman Reigns tested positive for COVID-19.
We don't know what comes next for Big E, but it seems unlikely he'll get a rematch. Why? Well, it's now being widely reported that Lesnar's first title defence will be against Bobby Lashley at the Royal Rumble PPV later this month. The Beast also isn't scheduled for tonight's episode of RAW, though he has officially been moved to the Monday night show after being drafted to SmackDown. That's just an internal decision, though, and it sounds like he'll retain his "free agent" status on TV.
It's still looking like the plan is for a champion vs. champion match at WrestleMania, but why WWE would unify the belts when there's a brand split is hard to say. However, that's a huge match for the Show of Shows and one that could drive up ratings in the months leading to that massive PPV.
Keep checking back here for updates.