Final Fantasy XIII-2 is being well-supported by DLC; it seems on almost a Harmonix-esque basis something is being released to either challenge, benefit or expose your characters. Some people will think that's a bad thing, but those people may change their tune in the near future. On top of remarkably cheap story episodes featuring popular characters, rumours for more remarkably cheap story episodes and aesthetic-only DLC like costumes, it appears that Square will soon be releasing "cameo" characters for the coliseum.
Siliconera reports that they've been told that cameo characters from other Final Fantasy games will be announced soon, characters like Gilgamesh from Final Fantasy V. They also say that a character from Final Fantasy VI will soon be announced, although they couldn't get word on who that would be. Once those characters are beaten, you can have them in your monster slot to use in regular battle.
If this turns out to be true, how long do you think it'll be until Sephiroth is released in glorious HD? I'm already on the fence about buying Final Fantasy XIII-2, especially considering how rapidly it has dropped in price, and, yes, some of this character DLC might actually push me over.