Bridge of Flowers:
https://en.wikipedia.org/wiki/Bridge_of_Flowers_(event)#:~:text=The Bridge of Flowers (Romanian,Romania and the Moldavian SSR.
I thought about a scenario in which Romania and Moldova would have united and this is it. On the 6th of May 1990, both Romanians and Moldovans crossed 8 bridges across the Prut River, and instead of being something symbolic in this scenario, it leads to the union of Romania and Moldova on the 16th of June 1991. The Romanian Army invades Transnistria before their ready to do anything and it gets exchanged with southern Budjak and Herta, there's a small population exchange between the Romanians living East of the Dnister River and the Russians and Ukrainians in southern Budjak. One year later the Romanian Monarchy was restored. Romania still joins NATO and the EU. It's something that just came to mind today so I'll be glad to expand it further with help. This is a map of what Romania would look like (in 2016).
View attachment 603586
If your POD is the Bridge of Flowers then first you would need a radically different Ion Iliescu, not a prudent pro-Russian, but a Helmuth Kohl-like ambitious person, able to ask for the Union of Romania and Moldova.
Secondly, it won't happen in 1991. Moldova proclaimed it's independence in 1991 since it had a favorable internal context inside the Soviet Union, with the uncertainty caused by the failure of the August 1991 Attempted Coup d'Etat. If it would attempt to proclaim it in 1990, it would be crushed by the Red Army, as in Lithuania:
https://en.wikipedia.org/wiki/January_Events_(Lithuania)?wprov=sfla1. As such, let's say that Moldova still proclaims independence on August 27 1991. Iliescu immediately begins moves to start the unification process, at least one year would take to solve all international and internsl issues(fixed leu exchange rate, same juridical system), so, in an optimistic scenario, the Union is finalized by September-October 1992, just in time for Iliescu to win in a landslide the general election. However, this is not over yet.
Thirdly, we have to deal with the Transnistrean issue. I have a feeling that seeing that Moldova was heading towards Union with Romania would make the separatists see the writing on the wall, so, after some small border clashes, after an Iliescu-Kravchuk-Yeltsin summit, Yeltsin would agree to withdraw the 14th Army from Transnistria and Kravchuk would agree to the territorial exchange.
Fourthly, the story behind the Union. This period was marked by a feeling of Russian-Guilt for the Soviet-era annexations and killings and a desire to imitate Western customs and institutions, so it's possible to get Kravchuk and Yeltsin to see accepting Moldova's Union with Romania as abandoning the Soviet relic of Moldova, which had lived up it's use. Iliescu would see the Union with Moldova as building a new Romania from the ashes of Ceaușescu's Kingdom and as cementing his legacy as Romania's first democratically-elected President. The Union is done, but here come the issues at hand:
Fiftly, the Russo-Ukrainian population of Moldova. Not hard to handle, Romania would replicate Estonia's Citizenship Law (
https://en.wikipedia.org/wiki/Russians_in_Estonia?wprov=sfla1) and Kravchuk and Yeltsin would both begin a resettlement program for their country's citizens.
Sixthly, Unitarian vs. Federal state? We're already so far from the POD that it's uncertain to predict which type of state this new Romania would be, since both options can be settled on. On the other hand, I feel like Gagauzia would not gain autonomy, since the Romanians fear possible Szèkèly indpirations for their autonomy plans, so Gagauzia would probably remain Comrat County in a federal or unitary Romania. Personally, I have a feeling that this new Romania would be a unitary state, since it never was able to have a federal form and most nationalistic Romanians preffered this system.
Seventhly, economic situation. Romania would need to impose a high solidarity tax, like West Germany had, to bring Moldovan income to the Romanian median, which would take a few decades. Moldova would, nevertheless, become Romania's poorest region, with only Chișinău prospering, as Romania's second-largest city. Internal migration from Moldova to the rest of Romania would be high. Overall, Romania would most likely have the same economic situation by now, with the addition of the poor region of Moldova to invest in being balanced by the population increase it bringed up with it.
Eightly, the monarchy question. Romania would not become a monarchy in the '90s. Iliescu has become the symbol of Romania's democratic rennaissance, the Great Unifyer, but he won't suffer a strong-enough hit to the head to become a monarchist. At most, he would welcome King Michael back in the country sooner, by 1994, but he would never, ever bring the monarchy back. Your best bet to bring back the monarchy is Iliescu staying in power throughout the ninties, ruining the economy and the Democratic Convention winning a landslide victory in 2000. This would enable them somehow get rid of the Republican Clause in the Constitution (Constitutional Court, separate amendment etc.) and organise a monarchy referendum.
Ninthly, NATO and the EU. An Iliescu that unifies Romania in 1992, is trustworthy enough to make Romania join the EU earlier. To make the best of your idea, in 2004 Romania joins both the EU and NATO and King Michael is crowned in the Patriarchal Palace.
Tenthly and finally, political alignment. Not much would change, PSD would dominate Moldova as it dominates most poor parts of Romania, the Russians, Gagauzs and Ukrainians remaining would possibly support the Right out of spite and have low voting turnout(similar to OTL Szèkèlys), along with Chișinău and the central areas of former Moldova.
This is the best I could make of your idea. Even if you thought of a different Romanian leader then Iliescu (Rațiu, maybe), to butterfly Iliescu you would need a POD at least in the '70s and it would no longer be mostly about Moldova and monarchy. Also, I think he would reinstate the monarchy first.
