Jim Rash was a great character until season 4 came along and they decided to make him an exaggerated predatorial gay steroetype. Hopefully Dan Harmon brings him back to the needy insecure litigiphobic bearaucrat who was also homosexual he was in the first three seasons.
They certainly do have enough material with the five characters who are left, but I also think they risk the dynamic getting stale, and sinking into the 'Everybody learns a lesson they forget by the start of the next episode' pattern of usual sitcoms.
That's why I'm in favor of bringing in other characters, so they can go into unexplored territory with their character relationships. Rich I think would work well. He's been established as this perfect wonderful guy with a dark desperation to be perfect. And Neal could be a buddy for Abed, he's already got depth as a smart, sensitive guy with self esteem issues and a history of being picked on by everyone.
And aside from people who could fit into the group dynamic, I really hope they bring John Oliver back into the fold.