Ok Im just going to explain an idea I just got for Sub-Zero that I think can work.

First it could start out someway of showing how the Classic Sub-Zero went through the process to make his soul tainted with evil (much like Sindel) when he went to the Netherealm. Since Sindel took 10 years to be completed with her tainted soul, and it's been 10 years since MK1 when Sub-Zero died, his soul could have been tainted with evil by Shinnok. Classic Sub-Zero now escapes from the Netherealm and starts up the evil Lin Kuei clan again and quickly allies Sektor and possibly Smoke (assuming he is still evil). They then try to start a rivalry with Sifu Sub-Zero's new and good Lin Kuei clan and they have a rivaly between the clans. That is my idea how they can bring back the classic Sub-Zero.