If you are going to start that late in progression you should move one more expansion forward and go with GoD so that players can start with any class instead of needing to wait for the next expansion to launch to start a Berserker.
I really thought I was going to like the first 51/50...The overall concept is a good one, for me where it failed is you started in full defiant armor. While Defiant Armor makes sense in the 80+ game it makes the 51-70 game boring.
How about GoD start, however, no teleporter to Natimbi. So they just have to do code, disabling the discord people sending you to Natimbi. Can't get to Natimbi can't do GoD stuff. So its GoD all classes but you can't do GoD stuff Natimbi onwards yet. They just edit the few NPCs that send you to GoD, not a lot of work. Commenting out that specific code, do you Natimbi, Natimbi option, aint gonna break nothing. Also, suppose you have to not have GoD portal spells too. Only a week of work tops.
None of what you said makes any sense, if you are going to open an expansion at GoD all of it should be available.
While I disagree with LDON's existence and starting that late, I am so sick of classic start that I would accept it with tears in my eyes.