While Capcom occasionally release new costumes for Ultra Street Fighter IV, PC gamers have been modding the various PC releases of Street Fighter IV for years, toying around with the character models to give them new looks. Costumes run the gamut from a Kratos outfit for Sagat to Sakura with long hair and a bandana.
Deviantart user Khaledantar666 is one such modder, and has been releasing a variety of costumes for both Ultra Street Fighter IV and Street Fighter X Tekken. You can find some of his work below with the appropriate links back to his Deviantart page.
A Bayonetta mod for Poison in Ultra Street Fighter IV.
An Iron Man Hulk Buster suit for Zangief in Ultra Street Fighter IV.
A Spider-Man costume for Fei Long in Ultra Street Fighter IV.
A Link mod for Cody in Ultra Street Fighter IV.
Other costumes Khaledantar666 has created include Batman and Batman Beyond costumes for Lars, a Vanessa look (from PN03) for Alisa, and an Alucard look (from Castlevania: Lords of Shadow 2) for Lars in Street Fighter X Tekken. You can find them all on his DA page.