Just because something "was once OU" is also not a reason to remove it entirely from the list of options.
You say they dropped from OU due to a "Metagame shift", well the metagame HAS sifted and we should work from how the metagame is now, not in early DP when people thought that Rampados was as threatening as Garchomp or Rhyperior.
An objective method would be to add in the 11 eligible Pokemon in BL for voting, and if they evolve Breloom or whatever its their choice.
That is very simple, all Pokemon that are not used a lot and fit the required things (not legend, ect) get to be in the poll.
Its our job to keep the mass from making something broken, NOT to tell them what is "more deserving" of an evolution.
Ditto won't be included for two reasons. One: like Unown, it only has one move. We won't be giving it a movepool, or else it just isn't Ditto. Therefore, it removes a whole poll from the process.
Not an issue, we just say in that poll: Should we keep the movepool the same.
Two: the stats are decided. It would have massive HP and Speed. Any other stats wouldn't help it in any way, shape or form, since it needs high Speed to get off the transform, and HP is the only stat that carries. Therefore, it would be broken, since you could easily transform into something like Gengar and hammer the opponent, while not taking much due to the high HP. Hell, transform into Shuckle and you've got yourself a gg there.
Surely we could find a way to balance its stats in such a way that it is nether useless or broken.... Saying we can't make it because it would be broken is... silly. With all the collected experience and knowledge we have, if we chose to, we could make an OU Ditto, or Unown (New ability anyone?).
But for Unown it is different, they have many different forms, they are depicted as a legend in the move so I can see you taking that off the poll.
Note that I do not support evolving Ditto in the least. I just think it should be on the poll for voters to deicide upon.