I would like multiple slots as well, i.e. command variant, dress uniform, etc. Although you can save the templates and load them up. Most of the uniform changes average about 500 energy creds, so that is a drop in a bucket at later levels. Actual slots would still be easier though.
Yeah, this feels like something Cryptic has on their docket, but is a low priority. They had to launch the game quickly, so just kept it to the one slot to avoid overcomplicating the system, but I know it'll eventually get changed somewhere down the line.
Actually beyond the default one slot and a second slot for joining a fleet I suspect they will be offering additional costume slots from the C-Store.
Champions Online C-Store offers the following microtransaction:
Additional Costume Slots (400 Cryptic Points per two additional slots)
Heroes have the opportunity to unlock three (3) Individual and one (1) Super Group costume slots by level 40. Players can purchase sets of two (2) additional costume slots to increase the flexibility of their hero! NOTE: These costume slots are applied to every character on an account.
So I can easily see that same microtransaction being offered to us in the STO C-Store. They started offering this just a few weeks after the launch of Champions Online so the same could happen here.