Taiwan (or The Republic of China) is a special case in that it doesn't want independence from the PRC*, but rather they claim that all of China is part of the ROC and is illegally occupied by the PRC. The PRC, likewise, views Taiwan as a rebellious little part of their own country.

*I learned just now that there's a political movement that eschews this view and instead just wants sovereignty for the land currently owned by Taiwan.

Anyway, as far as I know, Ukraine doesn't make any similar claims to Russia (except for certain contested areas).

As a pro-Ukranian Russian: fuck you. If Ukraine will try to conquer a single square inch of Russian land I'll volunteer to be in factory making tanks, despite being a trans girl