I am afraid I really do not like this idea. Later in the game, a larger group of smaller players is the ONLY way to overcome a larger opponent. We have already seen how the GT game mechanics encourage size snowballing. Limiting the number of alliance members only means that the alliance with the 1-3 largest players wins, rather than simply the largest alliance.
Also, on a map the size of Cerato or Einio, the $8k capital income is a pittance compared to total resource income. I saw early on that Fulgur, with its much reduced size, would be a different. Yes, EBM has a crazy number of people, but that does not speak to their cooperation or competence (many are newer and have not carried over from other maps like AO and EHJ); I still see EHJ, SWC, and EBM all having a fair chance at winning, and EHJ and SWC probably have the best chances because they are not in the middle (though admittedly the loss of Florida hurt EHJ).
I also do not think that limiting the number of players in an alliance is good because then you discourage or prevent established alliances from allowing any newer smaller players in, which will have the effect of preventing any newer players from joining an existing map since there is NO WAY they can possibly compete after a week or two.
Officially limiting alliance early-game is irrelevant because the alliance can agree to be one when it can eventually be afforded.
Oh yeah, and imagine that, if you aren't with the Communist Bloc or NATO, then you're boned in WWIII... In a wargame like this, unless you forced it to be FFA, it will always be the case that the game rather quickly boils down to 2-3 groups. The only way one of those groups will break apart near the end is if one of the larger members is such a dick he doesn't want anyone else to win with him and backstabs his alliance. Yes, this bones anyone who desperately wants to go it lone-wolf style, but do you really want to play a game that truly permits a player to win vs. all 20 other players working against them?
The carryover alliances are a bit of a problem. Friendliness, trust, and sharing knowledge are good, but mixing teams up a bit would also be good for the community, IMHO.
1) Doing something about carry-over alliances (who are winning a different map) might be interesting, but would also make it harder for IRL friends to play together. It is one thing to play against your friend for a 6-8 hour board game, but something else entirely to have to do it for months online.
2) For all those "Only ME!" people out there, an FFA, last-man-standing (well, maybe stop when one player has 51% of the land mass) map would be interesting; such a map would have NO treaties and NO alliances.
3) Another interesting alternate-rule-set map would be a map with pre-made countries (maybe even pre-made diplomacy -- alliances + treaties?) would be interesting.