What Craig said at the end seems a likely direction. I can give some tentative ideas, that should be taken only as that: none of this is definite and none of it is in my control. So, while I have a small amount of insider knowledge, I am not speaking for Wolfram Research.
(1) The Community will not in any way merge with Mathematica.StackExchange.com. It's not a possibility (they are independent of Wolfram Research). Even if it was possible it would not be a good idea for any of several reasons.
(2) It is certainly possible that the Community might subsume MathGroup. One motivation for this new forum was that it would retain similar Q&A features as the group, and a similar style of moderation, but would reflect the capabilities of the newer sites that have appeared in recent years (e.g. code sharing, which is in the works, upvoting, and more).
(3) (I hope this is not confidential in any way...) To the best of my knowledge, Steve Christensen is now also a moderator for the Community forum. If we do go the route of merging with MathGroup, I believe it will be a relatively seamless transition and one that has Steve serving in a similar capacity. Though perhaps (I should hope) dealing with less spam than at present.
(4) I'll be up front here and note that (3) above is not the complete story in one important detail. MathGroup, as run by Steve Christensen, has had considerable autonomy as an entity largely independent of Wolfram Research (though it is more closely connected than the StackExchange site). Clearly the Community forum is under the direct auspices of the company. This distinction has posed an issue at least in theory. In practice I think the main idea for both MathGroup and Community is to apply common sense to moderation. So the question of autonomy might not be much of an issue in practice. That at least is my hope.
(5) MathGroup also exists as the Usenet group comp.soft-sys.math.mathematica. On the one hand, I do not know whether or how such a group might be "subsumable" into a very different type of existence. On the other, my impression is that many Usenet groups are losing the popularity they had through the 90's and early 00's. While MathGroup remains fairly strong it is not obvious that it can continue in its current form indefinitely, as more modern forums are simply replacing the Usenet model. I guess my point here is that, independent of Wolfram Research vs independent control, there are external issues that both make merging tricky, and that argue for doing so in the not too distant future.
Vitaliy Kaurov knows more about the plans for the Community than I do. He is traveling right now, which is why I am attempting, in a perhaps clumsy manner, to address questions and concerns raised here. I'm hoping he'll post corrections within a few days if I made any major mistakes.