Natsu and Wendy aside, since Wendy is so amiable she could work with anybody, but Natsu and Gajeel are so incompatible under normal circumstances they wouldn't get anything done. They only manage to pull it off when the alternative would be a catastrophe for the whole guild otherwise. I doubt they could change that either without changing their basic personalities as well - and if they changed those, they might not be so formidable anymore. They basically kick so much ass because they are full of self-confidence from believing they are the best and strongest and nobody can oppose them. Two such people can't stand each other unless united by an unspeakable horror. Normal missions don't offer such adversaries. Gajeel seems to be a lone wolf anyway.

That might change once they grow up and become more responsible. Who knows. But in such a future they wouldn't really need to move around in teams.