Where do the Republicans go....

[TiltsAreUnderrated]

Assuming trends continue and accepting this thread's hypothetical that the entire sunbelt won't [just] flip back (which is questionable but worth entertaining for the thread's sake), I'd expect them to nominate a Texan in the hopes that a home state boost might counteract trends there. I think the first couple of Democratic flips at the statewide level in GA is also something Republicans could hope to reverse in the short term with increased turnout as a result of backlash/panic in the local base (it's incredibly polarised there), and that they'd focus on targeting ME-AL, MN, NH and RI. A relatively secular, economically moderate candidate who was protectionist but civil in tone might be best placed to appeal to these states, and they'd also be able to reach out to ME-01, NV (not wholly sunbelt) and OR if things were going really well for them.

VA is also worth targeting if the Democratic part of the sunbelt is to become a lost cause, but I doubt a non-incumbent protectionist Republican could reach it. If they're desperate enough about winning the presidency, perhaps they could come to support DC statehood on condition that it takes in some MD/VA suburbs and exurbs to make those two states more competitive for them. In theory, they could also back statehood for places that elect politicians from parties which are affiliated with Republicans, but I suspect territories like PR would quickly swing Democratic at the federal level once granted that.