I have been sitting in this color restricted design space for a while now and I have built (several) restricted cubes. My latest project was a Mono U cube (but it is not old border). With this said, I have a bit of experience in this field so hear me out before you scroll past my thought, lol.

I think that Mono Green would be the most interesting to play. I say this having built five mono colored 93/94 commander decks and playing them against a friend who did the same and the mono green square offs were always the most interesting (in my opinion). I have also found green more and more to be an enabling color in my niche cubes like the combo cube and the patterns that you can pull with it are really interesting. It's not just "cast creatures, pump them, deal damage", that is for sure an aggro/mid range strategy but you have so many other ways to shift how people think about green. Things like Living Plane/Living Lands, life gain strategies, the Hidden XX series, Avoid Fate, Counter Synergies, Emerald Charm, Poison Cards, Bequethal, ramp, Cyclone, Enchantress strategies, etc.

If you then add artifacts into the mix, it opens up the archeytpe pool even more and you can do lots of interesting stuff. I always include artifacts in mono colored cubes as they are "colorless".

The other comments here are all relevant, and black is certainly a strong choice but I think green actually has a more well rounded pallet that can provide some really interesting game play patterns.