Interwar Romania already had pretty much every piece of land Romania had a claim to.
There were bits and pieces that were still conceivable, as well as various opportunistic landgrabs.
1. Timok Valley
This is south of the Danube, roughly were the Romanian, Serbian and Bulgarian borders meet, and where there's a funny bend in the course of the Danube. By many accounts, there was an outright Romanian majority. Main problem is that it was part of pre-war Serbia, meaning there'd have to be some pretty convoluted circumstances whereby Romania could get this bit. Most likely scenario is the Entente big boys pulling out of the Balkans post-war for whatever reason, and then a war erupting between Serbia and Romania, likely over Timisoara (which Serbia had gotten to first following the Austro-Hungarian collapse, but was majority Romanian).
2. Serbian Banat
This is the bit of Serbian Vojvodina that's north of the Danube and west of the Tisza. Used to be part of A-H, so it doesn't have the problem above, but is much more mixed, with the Serbians likely having formed a plurality at the time. Romania lobbied hard for this at Versailles, but didn't get it. Have Serbia do something stupid, and they might.
3. Transnistria
Another place with a localized Romanian minority, roughly corresponding to the modern breakaway state. Entente could very well have agreed to Romania keeping it, but Romania didn't even try, as defending it from the Soviets would have required troops needed to conquer Budapest. Could conceivably have been gotten if some sort of deal was reached with the Bolsheviks early on, but I have no idea what that would have entailed. Alternatively, they could have snagged it later in 1920 on from the Whites if these were more successful (but not TOO successful).
now we get to parts that hardly had any Romanians...
4. Hungarian area east of the Tisza
Hungarians form a clear majority here. Romania claimed it needed it for security reasons, much like Italy had gotten south Tyrol all the way to the Brenner Pass. Was militarily occupied by Romania in the Romanian-Hungarian War. Have them be dicks and not giving it back after pulling out of Budapest
5. Transcarpathian Ruthenia
This was a sort of no-mans-land in the aftermath of the dissolution of A-H, and it took a while before Prague finally managed to assert dominance, only after the Hungarian bolsheviks were defeated. Had Romania chosen to occupy it prior to the Czechs getting there, they could have forced the issue all the way to the point of war. Given the overall unstable situation, and the ongoing conflict between Prague and Warsaw over disputed border territories, it's conceivable Romania could have hung on to it, either by the Czechoslovaks backing down, or by defeating them militarily.
6. Pokuttia
Romania actually occupied this bit of the West Ukrainian People's Republic, after both it and Poland had ganged up on it:
https://en.wikipedia.org/wiki/Pokuttya
Have the transfer be delayed for whatever reason, and then either a falling out with Poland, or worse a Soviet takeover of Poland, and Romania could decide to not hand it over, and instead keep it
now we're getting into the realm of the truly speculative...
7. West Ukrainian Peoples Republic
It's not inconceivable that a Romania that emerges from the war much stronger than OTL (say, by not entering it in the first place until late 1918) could try and force its dominance over this ailing would-be state, especially after something like the Lwow Pogrom happens. True, it would sacrifice Polish friendship, but Bucharest could rightfully rationalize it by the fact that Poland and Romania had a common enemy in the form of the USSR, and a common patron in the form of France, making any direct conflict less likely. If Poland somehow falls to the Reds, there's even more incentive by Romania to hold on to parts of Ukraine, in order for them to act as a buffer state.
8. Varna area
The only bit about this that makes any sort of sense is that Bulgarians did not make up an outright majority in the region, with Turks, Jews, Greeks and others combined forming a majority, if a narrow one. If the Entente decide to punish Bulgaria more severely for whatever reason, turning this area into either a Romanian puppet-state, or even making it an autonomous part of Romania, is conceivable, if only barely.
9. Greater Transnistria
This is basically the entire area between the Dniester and Bug rivers, including the important port-city of Odessa. While the western bit was majority-Romanian (or at the very least had a substantial minority), the rest was decidedly not. The only way for Romania to keep this would be by actively taking part in the Russian civil to a much, much, much greater extent, and then carving up this bit of territory at the end, more likely than not ruled indirectly via a White warlord or the remnants of one of the iterations of Ukraine (First Peoples Republic/Hetmanate/Second Peoples Republic/whatever)
10. Komancza and Lemko Republics
The only way it's even conceivable to think about these if Romania gains the WUPR from #7 as a puppet state, and then gets into a conflict with both Poland and Czechoslovakia, while simultaneously not having to care about threats to its east. If this unlikely set of conditions is somehow met, then some sort of meddling here is conceivable, though to what extent any attempts to exert influence here are successful is highly debatable.
and now for the truly sad and depressing...
11. Population transfers
There were quite a significant number of Aromanians throughout the Balkans in the 19th and early 20th century. If there had been a falling out between Serbia/Yugoslavia and Romania in the aftermath of ww1, some sort of forced population transfer, like what happened between Greece and Turkey, is not inconceivable. Lots of people displaced, lots die, overall Aromanian culture assimilated into the broader Romanian one, though probably slower than OTL since there would be more of them concentrated into a single area
en.m.wikipedia.org
With the Western border anchored northern border anchored at least as far as the Dniester River (pic 1) The western border anchored to the Danube (pic 2) and the southern border anchored to at least the south east Balkan mountains (pic 3)
