I actually think the next in line through traditional means would be Tyrion.
Nope.
Technically Gendry would be next using House Baratheons Claim to the Iron Throne (sort of? His claim to House Baratheon was derived from House Targareyen which would mean that in becoming a Baratheon he acknowledged that House Baratheon were unlawful usurpers).
Jon would be next using House Targaryen's claim.
Gotta remember the Lannisters were never actually invested with a claim to the Iron Throne.
It was Robert Baratheon ->Joffrey Baratheon -> Tommen Baratheon and then finally when there were no other legal options (with Stannis and Renly dead) Cersei Baratheon.
Despite the factual parentage of Joffrey and Tommen, legally all of that authority was derived from Robert's coup.
And after the Targaryen Restoration and with Jon exiled, Varys and Danaerys dead and Aegon being Sir Not Appearing In This Series no one left really had a legit claim outside of kinda sorta Gendry but again... Not really.
Gendry is a bastard though so has no claim. Daenerys took that away but had no authority to do so as she wasn't Queen. As far as the Baratheon/Lannister line was concerned he was still a bastard
Jon gave away his claim when he took the black the first time. Yes he died and came back which may technically get him his right back but good luck explaining that to people. Especially as all the people involved are now dead. Plus Jon also doesn't want the throne anyway
So Tyrion would be next in line I think through the Lannister family because even if it's derived through the Baratheon line, there isn't anybody else in line I can think of.
Yes for practical purposes no one throigh either claim has a legitimate claim for the throne.
So they either had to go find some random distant Baratheon relative/legitimize a Baratheon bastard like Edric Storm/Gendry