I don't see TOO much logic behind a CS300 order (unless it was mixed) but I see the CS100 as being a potentially good fit, A319 replacement, noticeably smaller than the A320 (CS300 has 160 seats, CS100 has 133) and thinking personally of Newcastle, I think it could lead to expansion. At the minute we have 3 A319, I could see 2 C-series, 2 A320/A320neo be a good move going forward. The C-series is right sized for domestic/EU city routes, as well as longer thinner routes, whereas the A320 is great for the high density, high frequency summer sun routes.
I really hope they do go cseries, maybe a split order for 100 with 50 each of the CS100/CS300, or just 100, 100's.
Much better economics than the A319/neo which is too heavy for loco
I hope they might go for 20-50 A321neo going forward as well. Gatwick, Luton and Milan would swallow 50 of them up no bother and I reckon would lead to a top up order.