I am facing the same issue over and over on legendary ME....it gets boring quickly after the midgame especially as empire...
The high elves, bretonnia and the dwarves are hunting down any form of chaos with doomstacks..... I will never have the opportunity to fight elite greenskin-armies or similar...all my lands are save and there is no reason to go to war with anyone, because I can just win via military alliances...
Now essentially I have to gamble to get a good endgame, meaning I must have enough "luck" that the armies I would like to fight actually win...which I can only figure out late.
I would like to have advanced campaign setting to boost certain factions, to make them initially stronger, more aggressive etc. to make sure that certain ones will survive and I can fight them.
ME was sold to us as "the ultimate sandbox", but without some kind of settings to adjust the experience, its just a coin-toss whether it will be epic or not.