An obvious choice might have been BRS given that Ryanair carried the annualised equivalent of 45K pax, so why didn't they announce that route at the same time as EDI and MAN?
AmeliaEarhart; I think your point is well made, however, I think that Aer Arann will take it slowly and not expand too greatly until they get confidence that LDY pays its way. In addition, given Aer Arann's previous experience with Ryanair they may not want to get them to react (although I realise they are expanding in some FR markets in DUB).
The fact that they are in effect designating a single unit ie ATR to the airport means that they will fly the PSO route and the other most likely routes that would be in demand and so MAN & EDI would be to my mind the 1st choices. No reason why routes like BRS, NCL, LBA may not follow.
I wouldnt be surprised if LBA pops up soon at LDY, maybe a 2 per week frequency but that's only an opinion.
EI-BUD