I think the thing thta stopped the AT-AT the most in GC was the trenches outside echo base, the AT-AT's always sorta 'fell' in the trenches and got stuck giving the rebels the oppurtunity too blow the thing too bits with rockets, I think that was a good thing tho, think the AT-AT oughta make it too about the trenches before it gets destroyed, its too powerfull too advance any further, and they always done a ton of kills and made the pilot very high up in the score board by the time it was destroyed.
As for the speeders, thats simple, include a couple of ties in the map, ok they wernt in the film but there was no reason for them not too be, cant see why the imps didnt lauch ties from the ISD's too deal with the speeders?! Just wanted an excuse too use theyre walkers me thinks!
Think the imp starting point shuld be a landing site, with AT-AT barges and AT-ST containers landed and open for the AT-AT's too spawn in, also a couple of imp shuttles would look neat as static spawns for the infantry, the tie's could spawn either side of the shuttles, makes em look like they were escorts then ;O)