I've read that getting a second J&J vaccine might do as a booster, and also that getting an mRNA (Moderna or Pfizer) shot after a J&J shot would do. (Though since you need two mRNA shots, a question to ask is if someone getting it after a J&J would need two.) Nothing I've seen has suggested it would be unsafe to get Moderna or Pfizer after a J&J shot.
That all said, since they are only apparently opening boosters to cancer patients and people with challenged immune systems right now anyway, maybe by the time they open boosters to the general public there will be better information on this question.
I read this morning that J&J is working on a booster and those who had that shot will be able to get it when it comes out. J&J vaccine came out a couple months after the others, so those vaccinated with it wouldn’t be able to get the booster right away anyway (the booster is 8 months after 2nd dose; single dose for J&J).
The article I read said that we should stay with the same brand that we originally had, but since Pfizer and Moderna are, both, mRNA, it might be okay to “mix” them, but that’s not known yet. If possible we should all stick with what we originally had.