I don't think it would happen time soon as the only real benefit is for the London Euston 390s as said earlier, which is pointless considering the amount of OHLE needed for a small number of Pendolino services a day.
Adding on to the ATW Electric idea, one thought I had was when 323s in the Manchester area needed replacing (which is some time away yet I hope!
) then you could possibly build a EMUD at Llandudno, Chester, Crewe or Holyhead and when trains needed heavy maintenance would use Longsight. The 175s Likely to be still around to replace or displace ATW 158s. This could be at least 5 years away, even longer.
So in conclusion, OHLE for North Wales would only be economical if ATW had a lot of benefit of it.