I think GLA/EDI/DUB are obvious choices (GLO's teeny terminal allowing!). Also it could also work if a niche player wanted to repeat the M2 success by offering a BRS/BHX alternative from the Channel Islands.
However, I don't see anyone taking the risk in the current economic climate and, as MUFC Fan says, if M2 were going to expand at GLO it would need to open a new base (with all the additional cost).
After everything M2 have put into GLO it would be a shame for another player to come in and take advantage of the potential there.