Thanks. Of course the best (for me) would be to store all addons on server - with layer based management.
It means saving tons of disk space over millions (hopefully) computers.(therefore Greenpeace will tell thanks for that, and we'll save loads of Pandas lives
)
And easy experience of each add-on without downloading whole content.
And as stated before, it would be most effective way for Multiplayer. As this problem exists in loads of Multiplayer games, where you had to download all modifications used by host to connect to it. (Then you find that ping on this host is way to high and trying to connect to other host, which uses other addons).
Of course the local addons also must be supported. As there wont be so much space on server for everybody and of course some modifications wont deserve this space.
There's another cool idea - Layer (add-on) mask editor. So user can mask an area of addon that he wants to use.
Imagine that you have 2 full world add-ons, but you really like Europe on one and Africa on another one. So basic layer management will not solve the problem, as one of add-ons will have priority on another one, and you'll never see it. Masking the areas will solve it fully.
All above will also work great not only for maps, but also for building, roads and some disaster things like forest fires and floods.
Actually biomes, weather, and traffic scripts also can be managed by layers and masks (especially masks), with gradual switch when you cross their borders.
Sorry daydreaming,
I'll stop here
