I've always wanted a MMORTS with millions of units on a huge map, but I just don't think it's resonable with current connection speeds.

Sins could work however, thanks the to modular nature of the maps.  Each star could be handled by it's own server and you would connect to another server by star jumping to it.  Sometimes you may not be able to get into a server, and the star jump would fail, perhaps the ships charge the jump for a few seconds but then just never leave.  You could come up with some silly physics theory to explain this in-game.

You could give ships to new players, for example, put up a handful of ships for a incoming player to adopt and use as his own, he would thus be your allie, atleast until he was large enough to oppose you. 

I haven't thought of all the details, but I think it's a cool idea if Stardock and Ironclad are ever looking for a really ambitious project...