We have competition laws and authorities which would probably need to approve any such collaboration. I doubt it would be approved on the basis that 4 carriers didn’t want a 5th joining the market. However they could probably codeshare or have some kind of frequent flyer partnership without much fuss.
Eastern seem a bit of a basket case. As soon as any other operator appears they drop the route, and it feels like their schedules change on a weekly basis so I’m not sure they add anything anyway. Probably the easiest carrier for Flybe to target but then Eastern haven’t made money for a while and Flybe flying bigger planes on the same routes will probably just lose even more.