With only 12 teams currently in the WNBA and salaries nearing D-League territory (minimum - $35,000, maximum - $107,000), I think it will be mighty difficult adding a third team to the New York/New England region.
The Sun was definitely created to be New England's team, but as the OP points out, there really is no association between them and the Celtics. This seems like a pretty easy fix if one team would reach out to the other. Having a couple of games a year at TD Garden would be great, but Mohegan probably doesn't want to lose out on any potential revenue.