Here's an idea for more costumes:
Have costume armor that makes your knight resemble characters from other popular videogames. Obviously it can't be too blatant, but I can imagine for example helmets that might be reminiscent of Link's cap (Zelda) or Master Chief's helmet (Halo). Or armor pieces that look like Samus's suit (Metroid), or Vincent's greatcoat (Final Fantasy).
Although maybe not the best analogy, games like Soul Calibur allow you to build custom characters that can greatly resemble other game characters; Mass Effect and other RPGs let you build your own facial features, which you model after celebrities and well-known people.

At any rate, more diverse costume gear would definitely make things more interesting. The prices can even be tiered; i.e. there will be some costumes available fairly cheap (~100e), while other costumes might cost up to 1000e. Along those lines, we should add a shield costume slot, and shield costumes.