After reading this i have a few comments to make;
1) Your quote, "There is very few things added to the mpq's, so making your old mpq's work with WarSoc shouldn't be impossible."
Isn't there a legality issue with this?
2) How exactly would you find existing games?
3) This relates to 2 and also the issue of lag. If you had an interface that allows you to find servers, you could also filter them by average ping.
4) This relates to 3 and also this earlier provided link,
http://www.gamespot.com/news/2005/09/02/news_6132467.html
It states that you cannot host the game on ANY server except for designated Battle.net servers;
"The 8th Circuit Court of Appeals in St. Louis ruled Thursday that federal law--specifically, the Digital Millennium Copyright Act--disallows players from altering Blizzard games to link with servers other than the company's official Battle.net site."
That right there is the end of your project. If your project is distributed i think it may come back to bite you in the ass.. Because Blizzard Entertainment is shamelessly greedy like all American companies and don't care what they have to do as long as they get money.
To conclude, as much as i like the options, not so much the details which seem to give the impression that map-makers are limited and thus subjected to certain learning abilities in order to use WarSoc for creating maps, i think you'd be better off creating your own game. You have that ability, the knowledge and the rare dedication. Or you could become part of an already existing gaming company.
There are free engines out there for you to start with, even my friend is creating or, by now, has created his own, and if you wanted you could probably hook-up with him for resouces or knowledge.
I really don't mean to put you off, but don't you think it'd be easier to just get a mass protest going, which would be equal to the amount of advertising WarSoc would need to become popular, pleading with Blizzard to implement these two options (and other declared options) into the WE? An option for more than 12 players, a bigger map size and possible joining-to-spectating-to-playing feature, which would do the same thing.
UMSWE has a map-size option of 480x128. To think of all the work you're going through, wouldn't it be easier to mess with UMSWE and find out what is stopping, or what would crash a larger map size and then modding it to work?
I remember someone said something about Bnet servers detecting map-size or file size? So perhaps i am suggesting an un-workable solution.
Yeah WC3 is a great game, minus the crappy, mindless, mind-numbing custom maps, but your potential is greater than this feature. Really, i just wouldn't want to see your potential go down the drain because Blizzard finally decided to do some investigating into their offspring communities and decided like exercising their influence and ruin your life because you wanted to help people have a happier experience.
Money rules in this land of wickedness, not the persuit of happiness.
I hope you do well in your endeavors.