I think this is a very good idea and can involve those in raid teams and those who aren't.
However, I would suggest running it in 10 man groups as they are easier to organise and as the loot drop is the same (almost) there isn't much difference. You could still all go in and fight for the area before hand (and use strategies via Vent) and then just split off into groups, pug some players if you have an odd few places and get lots of loot and have fun.
Although 25 mans are fun, when you are looking at an area which the Horde only hold for a limited amount of time at unpredictable times I think it would be beneficial to run 10 mans.
All that being said you may find that 25 people turn up and you have right set up to run a 25 man. It is all fairly unpredictable so it would probably be a case of come up with a date/time and just see what happens.