I really liked the concept of ninjas being fast, physical counters to magical units, given their pretty good res; while a good part of that was Fate's expanded weapon triangle, I'd really like to see that concept in other games. Of course, the theming would have to be different; maybe they could be called an 'Acolyte' or something, and have them promote to Dread Fighters, albeit somewhat nerfed from Gaiden and Echoes.
I'd also really like Griffon units to come back as well, preferably with its own line-- although I'd like some new mounts, too.