Well, if you're doing a fictional figure as becoming emperor, you might have something like:
Nicephorus Botaneiates doesn't trust/believe in Melissenos as much as OTL, so he writes a different letter. How that letter goes may favor that person more than the Komnenoi - perhaps there's an existing connection between them and Alexius so that the Komnenoi and Alexius's supporters) and George Palaiologos back them instead of Alexius, and we see them slip into Constantinople (no reason the "bribe the German mercenaries" can only work for the historical contender if necessary) in a way comparable to OTL Alexius I.
I'm not overly familiar with this period, so this is just a general idea - it depends on what kind of figure you want to insert and what role they've taken.
Maybe they're even the one Nicephorus III names (thus taking Melissenos's place as far as significance), and when they take the throne they make Alexius Caesar. How that goes or how long that lasts - well, that's more your call.