The 2 new potential fleet types (737Max or A220).
The 737Max would suck regardless, I’m totally biased. Possibly an entire fleet swap.
The A220 I could see happening (especially the -300) as its operating cost is less than a A319 NEO and it seats more (tied in with the cost structure).

Either way, dual fleets dilute relative seniority for those in the upper echelons of both seats...

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