@julyiscool: If they are going to space, why not pit them against the Main Man? Some mysterious villain has put a bounty on Goku's head and Lobo is here to collect. Since Lobo is such an inconsistent character, you can throw in your own spin to it. Maybe he unlocks the legendary Super Czarnian form whilst fighting Goku? They can punch for a bit, change hair colors, and blast the crap out of each other with energy blasts, and then be forced to work together due to the random arrival of another threat (Galactus? Brainiac? Thanos?).
...Or why not make some sort of Cosmic Hunger Games?