If Grendel had managed to leave his fight with Beowulf unscathed, he still would have suffered a defeat, and therefore it is likely he would have done exactly what he did in the text: return back to his mother's lair. This is what is described in the text before Grendel's mother's attack on Hereot occurs:
Broken and bowed,
outcast from all sweetness, the enemy of mankind
made for his death-den. But now his mother
had sallied forth on a savage journey,
grief-racked and ravenous, desperate for revenge.
It would be a logical assumption to make that Grendel, returning to his mother's lair ashamed and dejected by his defeat, would then seek her aid in slaying his opponent who had bested him. It is unlikely that a creature such as Grendel would be able to not return to fight Beowulf and to try and beat him, and therefore he would have done what he could to gain an ally in his battle and to ensure that he was better prepared and equipped to slay Beowulf the next time and not be bested himself. With his mother's support, Beowulf would have had a very difficult fight on his hands. This would have been Grendel's most logical next step had he returned from his battle with Beowulf unscathed.