Slitherine

Since there is a map editor, could not we use custom maps for our own battles along with random base maps?

Why not? But I can think of just one reason against it. The random generation algorithm ensures that the map is fair in the long run. You can still have some lopsided map setups on certain occasions but it's not the doing of the player.For user-made ones, there's no guarantee but the map maker must be conscious about balance.

Yes I've been thinking recently about cool designed maps would be too!

I think the current random maps are cool and provide a reasonable variety of terrain which usually looks fairly realistic.
Designed maps could bring some new things to the table; They could be symmetrical and wouldn't have to look like a real place, so could explore geometrical patterns and wierd geography that would create different gameplay situations.

The idea which is tickling my brain at the moment would be something like the famous BW map Heartbreak Ridge https://www.teamliquid.net/tlpd/images/ ... 0Ridge.jpg where you have a cramped high ground centre area with lots of entrances that has high ground spokes coming out.
The many transitions between high and low ground and having to balance flanking manouvers on both sides to tip the balance in the crucial centre region could create some cool situations.
Ultimately though I think it would take a lot of experimentation to figure out a good set of designed maps, but that could be fun!