I would say that at the minimum, the civilizations that they create would need to be mortal, sentient and consistent.
Mortal, for obvious reasons. If they are unkillable, then there is no way for them to lose (at least not easily. They could all be buried underground, but that would be a lot harder). They may be resistant to certain things, and would be adapted to survive and thrive in whatever environment they are placed in, but they need to have some weaknesses. Otherwise, if one of the players cannot lose, it ceases to be a game.
Sentient, so that the civilization can understand why they're fighting. It would not be enough to just make mindless beasts that would charge unfaltering into battle until they are killed. They must be made to understand why they are fighting (even if their god lies to them). This again levels the playing field, as each god can create a species that would be inspired by what that god is like (whether they get riled up by aggression, or submit to reasoned arguments), but would need a tactic to deter dissidence.
Consistent, because it would be too easy to make a creature that can simply transform into what it needs to be. If one god's transforming creatures came up against a herd of rhino people, they could just transform into something that can avoid their charge. This ties into the races requiring weaknesses, as it forces each god to make a decision on their creature's design. Their tactic is to minimize their weakness, not simply make them able to change what they are able to do based on the situation.
If they are that powerful, then the only way to make it a reasonable challenge would be to allow very little at all. This would likely include allowing inspiration, randomness, and time-constrained "miracles".
Inspiration, as due to the creature's sentience, they need to be coerced in some way into doing the bidding of the god. There would likely be restrictions on this, like not being able to appear in their true form in front of their people. They may be required to use a single person as a conduit - their champion - who must successfully inspire the rest of the people to do the bidding of that god.
Randomness, as there needs to be a dice roll element to the game. It could be like chess where there are strict rules and movements where there are no random elements, however when immortal omnipotent gods are playing chess against each other, eventually they will make every move there is to make, and it will stop being fun. This means that without a random element, they will eventually understand the strategies of the other players, and refuse to do anything to jeopardize their chances of success, so would eventually be stuck in a scenario where their only good move is not to play, so there would be a stalemate, ending the game in a draw.
Miracles, which must only be allowed every so often. This is so that each god cannot use their powers to interfere with other civilizations whenever they choose. They may decide to use their power to create a vein of pure iron where their civilization is digging, in order to allow them to equip their army with weapons and armor, or create a plague that wipes out half an enemy civilization over time. These miracles must be able to be mistaken for natural phenomena, otherwise like you said their choice of miracle could be to just burn the world. So the gods must think tactically, and use their choice of miracle to allow their people the best possible advantage at the best possible time, as they must wait a long period before being able to interfere again.
There will likely also need to be some sort of ratio of birth rate:life expectancy, so that gods cannot just create massive civilizations in a short period of time. If their people multiple rapidly, then they must also die quickly. If they live for a long time, then they cannot be able to produce 30 offspring a week. This would stop overpopulation, and make sure that there is a balance between the species.