There is a mod which will allow up to 150 on an A319. I believe there needs to be a slight change to emergency exits. In 'easy' config they would presumably remove much of the forward galley / wardrobe area to get another couple of rows in.
Whilst easy have, until now, gone for a single type, which obviously has a lot of benefits, there are also benefits with having 2 separate fleets.....imagine 75'ish 737's and the same number of A319's.....a lot of bargaining power between the two manufacturer's rather than being 'captive' to just one of the suppliers.
If they want to get into this dual-fleet position, with good bargaining power, then now is the time to start considering it.