Seeing how they are the remenats of the old U.S government wouldn't they be in a lot of Locations throughout the U.S like in the South or Texas or maybe they are behind the Commonwealth? Ever think about that.
This is true. To a degree.
Are they gone completely?
I would doubt it. We know actually know so very little about the Enclave and their operations that there could be a variety of places that they could still be hold up in. Given that they have inhabited Raven Rock (part of COG plans) I would also assume that they would have taken control of other COG U.S. bunkers such as Mount Weather or the unknown bunker of the United States Congress which is somewhere (was going to be the Greenbriar), and possibly even Offut Air force base (command center of USSTRATCOMM). So there are still many places where they could exist and logically would have retreated to given their defeats or even could have had a presence at from the beginning.
Are they weakend?
Yep. They've suffered some major defeats and their military strength is severely limited at this point. If they know what they're doing (and usually the Enclave do) they will retreat to yet undiscovered bunkers and hold up while they lick their wounds (which could take a very long time).
Should they appear in another fallout game? As more than just Remnants (like the ones in New Vegas)?
I wouldn't necessarily mind it, provided it made sense (the Chicago Outposts could be a good example). However if they appear in any near-future Fallout games as more than just nostalgic old men/women, they would need to be shown properly as weakend and should not just be "the bad guyzz" for us to kill (im getting tired of being forced to kill members of my favorite faction in order to "play the game right", goes for both Fallout 2 and 3).
The Enclave are an interesting faction (prehaps even the most interesting faction in Fallout, given that they are the shadow government of the United States) and I would hate to never see them again as more than the form of the "remnants" of new vegas. I just think it would be a waste to condemn such a wonderful faction to the abyss.