I'm a returning player and I don't know what team should i go with

depending when you started arena may be a long road to get to a reasonable rank.

You may want to check out a few you tube videos, they outline some good paths for beginners. some of the newer ones use separatists and take that path.

events really tend to be different factions, so there are not too many cross overs, but there are toons that can be used under multiple factions.

you will want to get a little work into your bounty hunters for the credit heist. that can be helpful at your stage.