I can very easily see him becoming a Jason Terry / Jamaal Crawford / Manu Ginobilli type - all three of those guys built extremely good careers out of dominating teams as the first guy off the bench, and you could rightly argue that every one of those guys was the second best player on their team during their prime years - despite the fact that they came off the bench.
During the Celtics big-3 (or big-4, if you prefer) era I really feel that we were one great sixth man away from being utterly dominant. If we had a guy like Terry, Crawford, Manu or Isaiah Thomas back then, I rally think we'd have won 2 or 3 championships.
I would absolutely love it if Thomas could carve himself that type of niche with the Celtics, because a deadly sixth man like that is probably one of the most underrated roles in the league, one that can absolutely turn the tide for a team.