Yeah, if they manage to secure the rights to Dragon Quest, I'm almost certain it's going to be a Castlevania situation where they have Erdrick as the original and Eight as the Echo. Luminary seems a bit recent, but we'll see.
I think the only issue with this is that when Sakurai mentioned about creating new fighters from Smash, it was mentioned, "And we're not talking about Echo Fighters." That's something that kinda hurts the idea of Echo Fighters being part of the pack.
Like, I don't see a problem with the idea of adding an Echo alongside a Fighter. Just that it was downplayed/made distant/whateverwordI'mlookingfor when they were talking about DLC.
That said, while the idea of Dragon Quest getting a "Hero" character that consists of 8 Dragon Quest protagonists sounds like something awesome, there are issues with this.
For one, there are certainly more than 8 characters. I mean, if you did a Hero for an Alt, when not including Nine and Ten's customizable character, you'd have the Descendent of Erdrick(DQ1), the Prince of Midenhall(DQ2), Erdrick themselves, Solo(DQ4), Madason(DQ5), Six, Seven, Eight and Eleven(the Luminary). Nine does have some Official Default designs though, much like Corrin and Robin. For Ten, I assume so as well, but I have never found their artwork. So that's 9(11 if you include Nine and Ten), so if they stick to 7 alternates, one(three) will be missing. Or if they get all 9(11), people may cry favouritism at a character getting more than 8 colours.
The 2nd notable one is the fact that there is technically a 10th Hero. In Dragon Quest IV, you could choose between a Male or Female. Solo is the Male, while the Female is named Sofia. Same goes for Nine, who has a Default Male and Female design. The thing though is that Female Nine's design is identical to Nine's Male design. Sofia, on the other hand, is drastically different from Solo and practically all of the Hero designs.
https://dragon-quest.org/w/index.php?title=File:Toriyama_DQ_Illustrations.png&action=view
https://dragon-quest.org/w/index.php?title=File:DQ4HeroFemale.png&action=view
All of the Hero designs are practically nothing but tunics and whatnot, except for the Descendent of Erdrick, DQ1's Hero. Sofia is practically Miss Gladiator and would certainly require a drastic model change for her compared to the other Heroes.
Then 3rd, the licensing issues.
Edit: Oh that's right. Dragon Quest III also had the option of being a female. But that being said,
https://dragon-quest.org/w/index.php?title=File:Hero3male.png&action=view
https://dragon-quest.org/w/index.php?title=File:Hero3female.png&action=view
She definitely resembles her male counterpart much more than Sofia does to Solo.