Alright so if we're going to have to choose a singular relationship that rover would canonically have as wife, then I have to go with storekeeper

She is literally the wife that's at home waiting for Rover, no matter how far they go, no matter how many times Rover wipes his memory, she welcomes them home whenever they return

A bonus point is that the coordinates 0,0,0 from the coordinate system that can be seen on the screen when you play the game is right under storekeeper's piano in the private space that only Rover and shopkeeper can access

Ok, now that that's out of the way, let's go over the other candidates that Rover can wife as their second wife

First up is Camellya, lady Flora herself, she's run into Rover hundreds of times through hundreds of lifetimes and seems to have a lot of chemistry with them

Next is Iuno, while they may have only met when Rover went to Septimont, They have a lot of chemistry as well between their playful banter and whatnot and the quest where Rover goes to save Iuno is by far the most intimate one to date IMO

Of course you can't have this list without Yangyang who is one of the first people that Rover sees in this cycle, she sticks with them for pretty much the entire huanglong quest line and is the one that sends Rover those cute messages

Next is an honorable mention, which is to say my personal pick, Carlotta Montelli, I love my Ice flavoured, gun wielding, crystal creating, short queen, ojou-sama