It will be interesting to see what happens.
Currently there is one Saturday morning slot free for an ORK based A320. There is also an odd-ball CDG flight on a Thursday, the only day of the week with 2 flights to Paris. I wonder if they would use that slot and the Saturday morning slot for a 2x week to somewhere, perhaps as 840 pointed out, Prague.
I think the A321 is being used on Rennes because it is sandwiched between two flights where the capacity is needed. The A321 is used to Malaga, Lanzarote and so on, on tuesdays, thursdays and Saturdays, happens that Rennes is operated those days. An A320 does the Heathrow flights on those days.