What a fun idea x) Only issues I can see with it is, well, from what I remember there was a counter that used to be on the FH home page that had a list of all the maps and how many players were in them at a time, the maps could have a max of 300 players in each map. So you wouldn't get far, not unless, you could count the other players in different maps as well.
Second issue would be lag. A lot of player's computers most likely couldn't handle the massive amount of players, aka love going around in that one map, all located together in one massive laggy pile. I don't know how much of a strain it could put on the server itself, but, I've never dealt with one so I wouldn't know. I wish I did so I could inform ya! I'm sure the server would be fine, though.
Nonetheless I would love to see this be a thing, granted granted it could be arranged!