Since you own Disgaea and Valkyria Chronicles you will probably like Fire Emblem: Radiant Dawn. One of the harder strategy games available, but totally worth the purchase.
FE:RD is the sequel to Fire Emblem: Path of Radiance on GC. If you can find it starting with that wouldn't be a bad idea. It is a little easier and you'll get the back story. If you can't get PoR, then starting with RD is fine as the story is explained really well anyways. The only thing to remember when you start RD is don't use your super powerful characters that you get off the bat, or you'll be screwed later on. Once your weaker characters catch up to them you can start using the stronger guys for something besides a shield.