The game should have options: Sandbox and Legacy sandbox.
Sandbox -> 1 character, same mechanics as warband
Legacy sandbox -> Self explained really. Aging, sons and daughters who claim the throne, extended diplomacies to support the mechanics etc.
Taleworlds forum is like one of those villages full of peasant old men and women who scream ''OH NO, OUTSIDERS, OH NO, WE DONT WANT NEW THINGS'. You guys should really go play your 90s games or something. You want to be old grumpy men, thats fine, the thing is nobody cares and no developer ever cared about the vocal minorities.
On topic, It would be cool if your son or daughter could be appearance-customized by you. Also, I think you should be able to have them as companion and manage the skills they learn etc. Or you can point them to have their own armies.