It will be one of BOH, LPL or SEN imo. Different cases for each of them.
BOH- Unserved area in Jet2's network. Supposedly they already looked at a Bournemouth base for 2020/21, but decided against it for unknown reasons. BOH is a better option for their massive 737 fleet over the runway restrictions at Southampton.
LPL- Manchester spillover. Very much a superior airport in terms of passenger experience. It seems to me that Jet2 may have reached their limit of expansion at MAN for the forseeable, at least before T2's 3rd pier will open towards the end of the decade. The airports are only about half an hour down the M56/M62 from eachother, so some existing staff would likely not mind the switch.
SEN-Most room for expansion. They have truly been decimated with EasyJet pulling out virtually everything, a huge gap there. I don't think it will infringe too much on their Stansted base either.
Outliers?
HUY is one that I haven't seen mentioned. It could likely support a 2/3 aircraft base, and is quite far from LBA/EMA.
CWL I personally think we are too early for that. Bristol base is only a year or two old.
The biggest outlier and surprise would be Manston. They want the airport to reopen to commercial traffic for early-2025. We know that Jet2 are very ahead in their scheduling, so what's not to say they have struck some sort of deal? MSE is a very underserved market. Furthermore, this likely wouldn't clash with a Hurn or Southampton base, perhaps even Southend.
LGW is too saturated to really get a good early footing. However, with BA Euroflyer supposedly considering a Manchester base, could this be seen as a warning shot from Jet2? The need for a Gatwick base would be eliminated if they wanted to go to Manston
IOM could be a seasonal 1 aircraft base of sorts, supporting a few flights a week to Spain
EXT is a boring case to be made. Same points as HUY really