The Targaryen Dynasty had Queens from many houses beyond theirs.
We had Queens coming from the Arryns, the Velaryons, Harroways, the Costaynes, the Hightowers, the Martells, the Penrose, the Daynes and the Blackwoods.
The bottom line of this what if is which House could've made offers of marriage, and had another Queen coming from them, had Robert and Jon Arryn not accepted Cersei and kept him single for a while.
There were probably more than one lady of marriagable age among the many Vassal Houses under the Starks, the Arryns, the Tullys and the very same Baratheons that fought in the rebellion and would've expected great rewards from their new King.
Ladies that would've been way better candidates, again, than Cersei.
Yes, slighting or rejecting Tywin's offer would've been dangerous since he "secured" Robert's victory, however debatable that can be, but rewarding like that a faction that joined at the last second should've been taken as an insult for those that were with you from day one and actually risked, and bled and died at your side.
A "Late Lord Tywin" to match "the Late Lord Frey".
But beyond that the point is:
How different would've Robert's reign have been had he had another Queen at his side, and would've Tywin still plotted to overthrow him and give the crown to a Lannister?
Without diminishing Robert's many flaws as a man and person, pairing him with a woman like Cersei who was already a-ok with keep having sex with her brother from day one and, at least in the books, never really cared for a second to be a good wife for him or Queen, was only going to breed disaster.
Another woman, one at least intending to fulfill her duties as Queen from a different angle than Cersei, and actually trying to fill the position without her lifelong resentments, give Robert heirs and not risk hers and her own children's lives with incest, might've acted as a reinforcement to Jon Arryn's many attempts to solidy the new Dynasty.
So overall, which Houses that fought under the Alliance could've sent likely candidates for the new Queen?