well you could wire up higher capactiy batteries, but you would need to make a custom shell / ductape because they will be larger than the original one.
but apart from that, what kind of mods do you actually want? i got a psp fat and the only mod id like is dual analog (right slider mapped to action buttons), but thats pretty much only for geometry wars. so yeah, as katsumbhong said there was never much need of modding a psp.