I think extended timelines are diificult to ballance. Also a good player will have beaten all the difficult countries and there wont be much of a challange left by the time you finish the current timeline.
Extended timelines work best for playing as a minor where a lot of time can be spent developing from a small country to a large one.
I am afraid you are right. It wouldn't be easy to pull off. Especially in a war strategy where conquest is rather a must than an option.
I am still hoping for game-play that would cover majority of 20th century. Perhaps by introducing countries under your control to have a chance to became independent again by different means...uprising, revolution or forced game event that would introduce and countries no matter how they're divided after you've won your conquest. Maybe this is more suitable for some experiential MOD than an DLC.
Cheers!