In addition to the aforemention current and upcoming stores, there is also Rook and Rogue. They sell used boardgames, so they aren't direct competition, but they are where folks want to go when they want to hang out and play a boardgame. There was also a short-lived gamestore that opened and closed this past year in Fairhaven. I forget the name. They were hoping their proximity to Fairhaven MS and their focus on Pokemon cards and kids-oriented RPGs would help create a niche. It didn't!
For my part, I don't go to DTG because it's the only game in town. I go to DTG because Nathaniel has built a great shop, slowly over time. I love the central location, and between the terrain, the tables, the ever-growing board game selection ... the store keeps improving. I think DTG could most improve in the area of family-friendliness. Staff who ignore or talk-down to female customers with money to spend (and who know more about boardgames than them) in order to play Magic is a big no-no.
With that said, I've never been in Cosmic before, so I don't know anything about their set-up, vibe, or amount of traffic. I'd look carefully at past business records if it were me. Even though you'll do things differently, it will take some time to overcome the past reputation.