Smash 5 will start the development cycle in the first half of 2017, I think... In that moment the NX probably will be in stock.

I think that after we get the characters from the Ballot on the roster that Mr. Sakurai should take a long needed vacation in order to recharge his batteries because working so hard on the same game for 3 years straight, non stop, it has to be exhausting for him, he's on the verge of being burned out. So he should take a long break, maybe start mentoring one of his closest working partners to take the reigns of the series while he's preparing to go.
Then after a few months off, and then when NX has just launched. He can start working on the next one, but this time, take on a more laid back role.
Instead of Director he can be the producer who watches over development, while he let's someone else take on the task.with him only chipping in every now and then
Then assuming NX is releasing next year, and development starts there Smash 5 may be in stores by Holiday 2019.

Despite Sakurai wanting the series to end, it sells really well each time so probably in a couple of years