I don't really think that will work. It will simply result in everyone wandering off, as per usual, then returning to some central location to complete whatever prereqs there are for the mods they really care about, before heading off and doing business as usual.
Quite frankly, I think the only way to achieve a cohesive center of everything is to literally place build restrictions on everywhere else. Outside of the very basics like a crafting table, dirt, sand, cobble, and other blocks which are used in exploring, just block all things off. No furnaces, no chests, ect. Have the designated 'town' area start out quite small; just a little plot of land (possibly z-level restrictions to keep it above ground?). Only about the size of the small spawn-hut on the current map. 1 chunk at most. Then, in addition to unlocking mods, the goals unlock additional land area. This could either be a N@W style chunk-claiming system (probably a bad idea, as different people will have different ideas for expanding), or better yet, a simple "+x blocks expansion in all directions." So at the start, it may be a 10x10x40 volume, expanding out and up as the server progresses. IMO, preferably up faster than out, as I believe that will lead to more interesting architectural feel; a super-high-density
Kowloon Walled City is more interesting than a sprawling suburbia of cobble crapshacks.
Though the main problem which will come up if we are successful at keeping everyone settled in one place: FPS drop. The place you and bread set up lags all to hell and back as it is; if people were told they had to live near that, I believe many, if not most, would quit before long. Measures absolutely
must be taken to prevent the excessive FPS drop created by certain mod blocks in order for this to succeed. Fluids, pipes, energy networks, smelteries; all those would have to be exceedingly carefully done, or forgone entirely, lest the map be entirely unplayable for people.