Where should Israel go back to then?

Israel doesn't have to go anywhere. No more than South Africa went somewhere after Apartheid ended in that state. Or Cuba vanished after the Batista government collapsed. Or Germany vanished after '45.

But the war criminals leading the current genocidal policy towards the people in their care. They need the same treatment as Milosoviec, Omar al-Bashir, and Saddam Hussein.

Drag Netanyahu to the Hague and prosecute him for crimes against humanity. Dismiss the current illegal and unconstitutional Kinesset and hold new free and fair elections in Israel. Provide reparations for the displaced Gaza and West Bank citizens, and launch a Truth and Reconciliation Committee to recover damages for the crimes committed against them.

That is the only moral path forward for Israel.

Where should Israel go back to then? What other land does it have claim to where it should stay?

Israel isn't a person. It can simply dissolve into nothing.

The people who moved there to take part in the land stealing can simply go back where they came. Or they can stay and live alongside the people they tried to genocide as equal citizens.

I'm sure the US government will be delighted to harbor all their war criminals above a certain station.

Not the person you're responding to

Obviously Israel shouldn't exist at all. It's not a diplomatically expedient thing to say, but it's the only real anti-colonialist answer because Israel is actively a colony. That doesn't mean expelling all the Jews from Palestine -- it might shock you to know there are non-Israeli Jews currently in Palestine -- but it does mean that the proper boarders for Palestine are restored and thus that there is no ethnostate (Jewish or otherwise). Recent additions to the Israeli colony (for they are always recruiting) can be sent back to their homelands. Many of them have been there long enough that they have no connection to those homelands anymore (including the more direct descendants of Holocaust survivors, etc.) and they can stay on the land that was Israel, but a lot of them are only there because they like the Jewish ethnostate thing and will therefore flee of their own will. That volume of people fleeing will probably incite still more to flee, but some will remain and they should be treated humanely and as equal citizens, prosecuting whatever crimes they committed but not being treated like subhumans the way that Israelis treated Palestinian prisoners.

The US is a much more difficult question because unlike Israelis being a minority compared to Palestinians, there are many more Euro-Americans, etc than there are Native Americans because the genocidal project of the US is much more complete. Inevitably some land, the land recently taken and some of the more useful land than what the Natives have been left with, should be returned to them, but obviously deporting all the Euros, etc. is neither viable nor productive, like how (to a much, much smaller extent) deporting all the Israelis is neither viable nor productive.

That said, separate from demographics and land rights, the US government absolutely shouldn't exist because it is terrorizing the world, including through proxies like Israel.