As a designer here at Discourse, I would say that creating an entirely new front end would not be reasonable or worth the effort. EmberJS while different, is very much like most front end frameworks in the way it works.
The emberJS community is also pretty helpful and vibrant as well. Id suggest using the time you would have devoted to building a new front end with NextJS to learn EmberJS 
After that, you’ll have a knowledge of two frameworks AND the ability to create custom themes & theme components for Discourse forums.
If that works for you, I’d start by taking a look at this popular & helpful topic.