Genoa or Rome is my guess!
Genoa is not a Ryanair base and Red Flyer says the route will be operated by a non-based aircraft. It's not entirely clear whether it will be a new route for the airport or a new route for Ryanair from BRS.
So far as I can determine these are Ryanair bases in Italy (including autonomous regions such as Sicily and Sardinia).
Bergamo - already operated by Ryanair at Bristol year-round
Bologna - already operated by Ryanair at Bristol in summer
Pisa - already operated by easyJet at Bristol year-round
Rome FCO - already operated by easyJet at Bristol year-round
Rome CIA
Palermo
Catania - already operated by easyJet at Bristol year-round
Naples - already operated by easyJet and TUI at Bristol, the former year-round
Milan Malpensa
Bari
Trapani
Cagliari
Lamezia
Alghero
Brindisi
Pescara
Ryanair has operated these Italian routes from BRS in the past:
Cagliari
Treviso - moved to Venice MP apparently to compete with easyJet
Trieste
Turin - operated by easyJet and TUI in ski season from BRS at present
Rimini
Cagliari was one of a number of routes operated by Ryanair from BRS in summer 2009 at the time of the recession, not all to Italy. I wondered if the airline might try one or two again (easyJet is actually bringing back one of them itself this summer - Montpellier). Cagliari might be an option - bmi regional operated it for a tour operator in summer 2017 - and so might Rome, especially as Ryanair seems to be duplicating a number of easyJet routes at BRS, although Ryanair seems to operate its UK flights from Ciampino whereas the easyJet BRS route is to Fiumicino.
Bari is the other that might seem a possibility. I would also have suggested Milan MXP but is Ryanair likely to compete with itself at Bergamo? Having said all this it will probably turn out to be somewhere else. It's all all good fun guessing though.