I know you already voiced your opinion on Carzel (Carla x Marzel), but what’s your opinion on Elena x Marzel?
Elena x Marzel seems cute. They're both stubborn, they both love their families, and they both are learning how to be good rulers and doing a pretty good job so far. All the scenes with them interacting are always pretty fun, so they seem compatible. They both had their biases about each other at first, but they became good friends pretty quickly. Elena taught Marzel a lot about land stuff (and those scenes were always hilarious). Plus, he gets along with Francisco, which is an added bonus.
Politically, it probably would be beneficial? It would likely strengthen human Serena relations. Although geographically I don't think they'd be able to merge Avalor and Coronado, so they'd have to figure out which of their possible heirs would inherit each Kingdom, but that probably wouldn't be too much of an issue. I definitely don't think either one would have to move to a new location permanently. (Elena is so magical now I feel like spontaneously getting water breathing powers would be expected at this point.)