Expanding the border wouldn't do a whole lot since most new people build near the spawn/warps so they just continue to get cluttered. Not to mention theres still a lot of space in the member world + a whole new guest world that's still fairly empty. And if the borders got expanded a few people would prolly snatch up giant chunks of the new space with ps and we'd be back where we started
As for more mining space.... Even if you're digging tunnels 1k blocks long... the world is like 6000x6000 so you'd need about 2000 tunnels each 6000 blocks long for every 2 block levels to effectively mine out the entire world
That's 24,000,000 blocks for every 2 block lvls of tunnels
So 668,000,000 blocks before you can mine out every ore from bedrock to sea lvl
Dunno about you but I've only mined maybe 100k blocks in my lifetime
Thing is, if you want more space you're gonna have to try harder to fill up the existing space otherwise there's no point in adding more.
Side note: personally I'd rather wait for 1.6 and just get a whole new world since then we'd have acess to all the new content + by then I'd assume there'd be quite a bit less space