#3 Showing up in some capacity at TakeOver: Tampa
Obviously, this scenario is very similar to if he shows up in some fashion at TakeOver: Portland. The card is stacked itself with six matches instead of the usual five matches. In order to have an impactful debut, his emergence could be a part of the aftermath of one of the matches.
Again, he could also show up in the crowd. That, however, wouldn't have the same effect as attacking someone backstage or after a match would have. Someone with Kross' character needs to debut in a violent manner. With TakeOver: Tampa being the last show before Mania, his WWE tenure could easily begin by taking someone down the day before WrestleMania.
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There could also be some sore of scenario where a performer has an open challenge or a match with a mystery opponent. It all boils down to how and when they want Kross to debut. Since he's been off of TV since last summer, I think it would have the most impact in an adversarial way. The manner in which Keith Lee has risen over the last year would seem to make him the perfect target for Kross should he start out in NXT. Will his potential debut be one of the memorable moments from WrestleMania weekend 2020?