End of episode 1 plot twist: the hero joins the Demon King.
How havent we had that yet? The closest we got is _Hataage! Kemonomichi_ where the protagonist straight-up refuses the Princess' request, even physically attacking her before running away to do his own thing.
Pretty sure with the copious amount of isekai LN out there, quite a few might actually did that already. They're just less popular than the ones that play it straight.
Although it's not exactly that, one example is Hiiro from Konjiki no Wordmaster, who does refuse the king's offer from the first chapter and he does team up with a demon lord later on. Like hundreds of chapters later on. Also the demon lord is mad cute.
