I would not be surprised if the devs implement world generation options in the future. Right now, they are developing Milestone 2 of their Early Access game that came out in February. So I think it's really just a matter of time, but exactly how much time? No idea. Their road map has been tweaked over the months due to the amount of players giving feedback.

Alternatively, while there is no map editor, you can use https://valheim-map.world/ to help generate a seed more to your liking.