New types of map blocks are perfectly possible. No coding required for that
You'll just have to add the filename of your new mapblock to the list of mapblocks the system has to choose from, for the specific terrain type.
The link between globe texture/mission type and terrain type(s) will become a ruleset too, so you could add new terrain types too without coding. I will foresee the ability to have multiple terrain types per globe texture/mission type as well.
So you could have one globe texture "forest", which in vanilla will load the forest tileset and forest maps if a UFO lands there. But you could add to the globe texture "forest" maybe a terrain type "forest with barns" that loads the forest and barn tileset and uses new mapblocks that has forest with an occasional barn in it.
I hope that gives some ideas about mods you don't need to do any coding for. Two UFOs in 1 map is just a little different, it affects other parts of the game, like the geoscape (alien activity for the region, what if you abort a mission, etc)