Actually, at some point its no longer Negima.. its a different thing using that title. I'd reckon just from screen budget logistics they'd immediately pare down to a core group of actors (the rest just wandering through on cameos or for special purpose missions).
All you can hope for in terms of the actors is personality and some general attributes (size, build, expressions .... ). It'll be interesting to see how close they get (pauses to zone out thinking of Kaede and Mana).
Actually I remember Ken A. going on and on about the VA selection process (he obviously enjoyed that) -- makes me think this is some grand schtick so he can interview hundreds of cute actresses
All the arcs have big budget effects (Eva, Kyoto, Tourney)... and again, I just don't see a live-action japanese television budget pulling that off. So if you think, scale down..... Negi vs Eva, collecting basic pactio girls in small events, perhaps him training with Eva&the others - seeking Nagi (or something important), but character-driven rather than big-action.
Okay... so who's going to play a 10 year old English Boy and carry the weight of the series?
That thought has me thinking perhaps Negi would be peripheral and the story focus might be from the girl's perspective.
Just brainstorming here while waiting for a process to finish. Technically, this is still just a rumor -- just well-founded so far.