Two icons, one accolade Fernando Alonso and Robert Kubica are the only drivers in the 21st century to win both a Formula One race, and the 24 Hours of Le Mans.
(sh.itjust.works)
Likeliest I think is GT3 cars, he really loves those both IRL and on iRacing. Would be absolute box office and a huge marketing win for the series if he rocked up at GT World Challenge or DTM.
Otherwise yes, it wouldn't surprise me if he did WEC hypercars too at some point. He's talked about teaming up with Alonso before, so he's definitely thought about it.