I would say it would really be a mod from the persian wars at the very earliest to the RTW vanilla end date.

I only really would say that major mod teams like EB or RTR, say, donate some of their 'best' stuff, not much, but say 5-10 units. Then, individual skinners could contribute maybe, and also add a couple of popular maps to it.

I understand that some stuff may not go together, and the solution to this would be to select work that does. Looking at the atists gallery on TWcenter, it seems clear that with the contribution of only a few people there you can have a great finalised result

Also, if this did occur, it would hopefully be a 'community's' mod- one for all to help if they wish, and hopefully one free from any sort of rivalries/hording away work, as i would hope a community mod like this would simply allow anybody who has done anything worthwile in the community to add it...