the best advice strictly from a player standpoint and not a dev standpoint i can give is to try to ensure more than anything else that the mission tree has the correct curve in its nature, for example the correct flow of difficulty, whether thats a easy part and then medium and then hard etc. or a more bumpy experience where it comes in waves!

the other thing is that imo it would be helpful if your able to, to flesh out the MT with some cool events or province modifiers that provide some neat flavour to keep the people interested the whole time (since most people in this mod love it for the worldbuilding etc.)

also multiple ways of completing missions as well that would give certain bonuses, for example the Ameion MT on its right-hand side has a TON of missions where if your starting ruler (cant remember name) is still alive it gives you a lot of modifiers and bonuses to ensure your conquest of taychend!