For those who support reunification, what should happen to Gagauzia? What should happen to Transnistria? What should happen given likely strong popular opposition from both of these regions?
Gagauzia : probably no real opposition, just the opportunity to try and get more autonomy.
Transnistria is a mafia state so it's not a question of national autonomy. When it comes to the rights or Russian and Ukrainian speakers I'm pretty sure Ukraine will lobby hard for the latter to be respected and the EU would try to get Romania to respect Russian speakers so as not to cause a casus Belli
Romania respects minorities (with the notable exception of Roma people) far better than most countries though. Dedicated seats in parliament and their own goddamn president is from a national minority, as corrupt as he is.