#2. Possibly getting the rub over The Fiend and Roman Reigns
When they say "Be careful what you wish for", the scenario has a high chance of applying here. As mentioned, the two biggest names they can go with as Braun Strowman's potential challenger are 'The Fiend' Bray Wyatt and Roman Reigns.
Roman Reigns would be the logical rivalry since it's a renewal of their fantastic feud in 2017. They proved to be perfect opponents for each other and are an example of two superstars who elevate each other while competing in the ring.
Fans would have no problem with Strowman beating Roman Reigns, but there's a high possibility of him facing 'The Fiend' Bray Wyatt as well. If he beats Wyatt, the reaction won't be good, even if it's not as bad as when Goldberg won the Universal title.
'The Fiend' is the most protected character in WWE and even if he loses to John Cena (which you'll know if you're reading this after day 2) at Wrestlemania 36, we still expect him to challenge for the Universal title soon.