i think you're talking about 2 different things, somberSun about placing additional content in specific yet unused/blocked map regions, Davor about removing _all_ world borders (like plenty mods already did for prev games).
so, from a properly worked out specific region beyond current borders, you'd not get that feeling of emptyness you get when you walk through unlocked, but unmodified areas (that weren't meant to walk through, but just as a kind of 3d background image behind the borders)
from a modder's perspective now, all these cells are, apart from being blocked, totally normal, valid game cells that could be used to do or add whatever.
but: in previous games, with more than 1 mod installed that unblock a specific region, only the border changes from the last one in load order would get taken over in the game, meaning if mod 11 unlocks a top left corner and mod 12 a bottom right one, you'd only be able to access the latter, while the first would be back to blocked.
for that reason, you could either take that risk (bad option), or unlock _all_ blocked regions even if you only needed just a tiny part of it, or, well, build your stuff somewhere else
so, if they just made the way cells are blocked a _bit_ more flexible (can't say much before geck's released), it'd well be possible to do mods (or dlc) beyond the current map borders, and some things about specific sights beyond the map borders give me pretty high hopes that just this has happened and they're planning on on-map dlc themselves. might also just be wishful thinking of course though.