You will always have mountains on the 4 edges of the map, that's not something that can be altered. The best you can get is the WaterWorld maps from ColonialCharter.
You usually get one HUGE lake covering most of the map, sometimes with an island in it. Forces you to think and plan how you're going to build as there's very little land available. Good thing there are dock buildings and stuff to build on the lake!

Thank you