There is more of a challenge in optimizing a PrC that is generally regarded as poor. However, coming up with an original concept for a popular PrC is just as difficult.

There are quite a few Core PrC's that have not been touched. Horizon Walker was mentioned. Assassin, Blackguard, Duelist, Dwarven Defender and Arcane Archer could make for a fun challenge. Some of them are often dipped, others rarely touched in many optimization guides and handbooks. (AA, DD and Duelist).