How difficult would this be? NEW GAME +

Oh you won't need SRAM expansion for this. You just need 1 new event bit (to indicate that it's a NG+ playthrough), and there are plenty of unused ones. As far as event code, you could do the NG+ check in just a handful of event bytes.
4 bytes inline - add a subroutine to the ending cutscene that inserts the dialogue asking if you want to start a NG+
14 bytes ($0D) - The dialogue to ask if you want to start a new game plus.
9 bytes - Set an event bit, use the event command, and branch back to the opening cutscene.

If you are working off of Divergent Paths, there are several sections in the event code where you could squeeze in a 14 byte and a 9 byte routine. If you are using vanilla, there are 18 bytes available by default, and it is not hard to free up 9 bytes somewhere else. For example, by overwriting some of the unused scenes.