Trump has been hollowing out the Republican party and filling it with his minions for the last 8 years. If they lose this election badly enough, I wouldn't be surprised to see some intense infighting and a big push to clean out a lot of Trump's people. While it's not inconceivable that you could see an offshoot try to replace the Republicans, I doubt it would happen. I think it's more plausible that the die hard MAGA idiots that can't read the room will get purged in favor of those who can at least appear to be more traditional, establishment Republicans. The party changes (as they do over time) but it doesn't go away.

Of course, I think the most likely outcome is that any rise of more moderate Republicans isn't going to be sufficient to fully dislodge the MAGA loyalists and you'll just see a much deeper divide. Without pressure from Trump constantly crushing them down, the more moderate side of the party can probably be more effective in uniting against the extreme right wing, but not enough to fully take over.