I see a lot of talk of "A team" and "main team" on Reddit... Do people really use the same team for events in different realms? I always build a team from scratch focusing on realm synergy. I only include off-realm characters for a good reason, like using Tidus w/ BSB and OSB against a boss weak to water, or if whoever my healer is for that realm just isn't up to snuff and I have to sub in Y'shtola or Aerith instead. But I'd say I clear 95% of content with a team that is either 5 on-realm characters or 4 on-realm characters + Tyro.
I rebuild a team for everything, though I probably only use 25 or so characters with any regularity.
Either way it comes down to the SBs you have. E.g., the U++ in the upcoming event is weak to lightning, so I will bring Ashe, since I have her SSB.
The only character I really use outside their own realm that I don't have a SB for is Yuffie.