Carol I was the pro-german king of Romania, but he died in 1914, let's say he lives longer and in 1916 Romania joines the Central Powers.
What effect would this have on history?
Do the Central Powers have a larger chance of beating the Entente?
Not happening unless a CP victory is clear, in which case we'd try to grab Bessarabia (and maybe Odessa) while there's time without irritating the French.
It's more likely to go to Bulgaria. Also, as long a there'll still be a Serbia it'd be wise to avoid the possibility of conflict with it (except over the Banat, of course).
Romania is unique country-it makes more problem for its own allies, than for its enemies. Russia would send some troops in Romania, as in RH . CP would have to send the same forces here too.