Greater Romania
It wouldn't have made much sense for Romania to fight alongside the Central Powers as one of its major war objectives was to gain Transylvania FROM the Austro-Hungarian Empire.
At the time Transylvania was over half ethnic Romanian, which was actually down from over 60% in the mid 19th century (due to many demographic processes ex. emigration & also a policy of 'magyarization'-hungarization). Either way even Hungarian historians agree that Romanians have been a majority at least since the 17th century (Romanians believe they've always been a majority). This is one of the 3 historical Romanian lands, and uniting it with Wallachia and Moldavia (which joined already in 1859 to create Romania) has been THE objective of Romanian nationalism since its very beginnings.
A victory of the Central Powers with or without Romania would have made the union of Transylvania with Romania impossible. Even if the Austro-Hungarian Empire would have suffered some kind of implosion, regardless the victory- which is probably a scenario worth exploring- the Hungarian Kingdom would have likely gained independence and preserved the unity of Greater Hungary.
Romania, as an ally, would have likely received Bessarabia- the Eastern part of historical Moldavia, occupied by Russians in 1812. But that is hardly comparable to Transylvania in any way (population, area, wealth, national mentality importance etc.). As it happened, for the short period between the two world wars Romania got both Transylvania and Bessarabia plus a number of smaller lands like Bucovina, thus, despite the initial military setbacks, joining the Entente paid off as much as it could possibly have had.