If you are going to carry on a belt, and you definitely want a 45, I'd look at the Glock 30. It is somewhat of a thick gun, but can be carried IWB, etc, and then you can also have Glock 21 mags as extras/reloads.
If pocket carry (with a proper pocket holster) is a real consideration, then you might look at the XDs.
Personally if you are going to carry on the belt, I don't see the benefit of the very small pistols such as the XDs that give up so much capacity to another decent sized gun that you could put on the belt. However, that is my opinion, and you really need to see what sized gun you are comfortable carrying in a quality holster/belt combination IWB, etc.
If you look to change calibers that opens up a bunch of other possibilities.
If you are not going to stay with a platform (i.e. same mags, holsters, etc as an existing pistol) then changing caliber now may not be that big of an issue, especially if you don't reload.