Except here are some of the issues:
-Dragon Quest is overwhelmingly more popular in Japan than anywhere else. Roughly 93% of the series entire sales have come from there.
-Dragon Quest from a marketing standpoint makes no sense because they've already tried to sell the series in the West multiple times during the series 32 year history. The only million seller was the original on the NES. They tried on the NES, DS, Wii, and even 3DS and the results were always yawn inducing. Why would the Switch be any different? They've tried new installments. They've tried ports. They've tried spin-offs. They've tried casual games and nothing has stuck. Another spinoff and a port is going to change this? I sincerely doubt it.
-Sakurai has repeatedly talked about the importance of international appeal for characters and people actually wanting them.
http://www.sourcegaming.info/2016/02/23/nintendo-dream-interview-with-sakurai-part-2/
https://www.sourcegaming.info/2016/01/20/sakurai-x-nomura-creator-interview-2016-part-one/
Based on this, the last totally unique character and franchise rep that got in solely because of Japanese demand was Marth and that was 1. 17 years ago and 2. such a controversial decision that they honestly debated cutting Marth and Roy from Western releases of Melee.
-In this day and age, the world is a small place and it's not hard to find things about what Japan wants in Smash. Characters like Chrom, Isabelle, Inkling, and K. Rool have mountains of evidence on their side. Even Joker support isn't hard to find, it's like trying to find your friends in a corner booth in a largely empty restaurant. Dragon Quest in Smash support is like trying to find the custodian in an empty stadium.