Romania would need to have a neutrality favorable to the Germans still. They need to promise to give the Germans as much oil as they can provide. They still need to allow transit of German troops. They still need to cooperate as much as possible with lots of goods exported to Germany essentially for free (in theory, paid in credit due after the war) to assist German production efforts.
There is a chance Hitler would be willing to accept this. Neutral Romania means less chance of an Allied strike against the oil fields. If he attacks, the Romanians could destroy a lot of the Ploesti fields. Hitler doesn't think he needs Romanian manpower during Barbarossa (though this will change by 1942) since he expects the Soviet Union to collapse. By 1942 though, Germany will need every man available to fight the Germans and may not have anything to spare for an invasion of Romania.
Romanian neutrality will have mixed effects on Germany. They may actually improve their oil situation until mid/late 1944 when the war would be doomed anyway. Without Romanian armies though, the Germans will have less manpower on the Eastern Front. That hurts, but it also prevents a situation like at Stalingrad when the Soviets specifically targetted a Romanian army as the weak point for their attack. The Germans will look for other ways to make up any manpower loss. Perhaps an earlier expansion of the Waffen SS to other countries. Romania might even send volunteers to the Germans like the Spanish did with the Blue Legion. It would be a good way to get rid of all those Iron Legion types.
Romania will not defect to the Allies until the Red Army comes close to their borders and it would be impossible for any German invasion. At the very least, both Romania and Bulgaria stay out of Communist hands.
A Romanian attack on the Axis would probably be to regain the lands lost to Hungary. It is even conceivable that Hungary, knowing the war is doomed, might prefer to make sure Romanian troops occupy them before the Red Army comes. So possibly even Hungary might stay outside Soviet occupation.
Depending on how the timing of the Romanian attack plays out, Germany may lose the war before the end of 1944.
There are lots of other scenarios possible. I consider the above one possibility, not the only one.