Nintendo knows the DLC will sell regardless, this is what the season pass exists at all.
I don't think the main goal here for the DLC was prioritizing DLC sales, but instead strengthening relations with third parties.
Keep in mind, Sakurai said that Richter/Simon were popular on the ballot, and they aren't DLC.
That's just my personal opinion, and I think that stands regardless of which SE rep we get.
It honestly plays largely in Erdrick's favor while simultaneously playing in the favor of possible future SE reps. Viewing this from a relation perspective, Nintendo and SE have had the rockiest relationship between company and third party. That being said, we could apply this to most any SE rep except possibly Sora, given he's more Disney than anything, and Sephiroth, who already has rep in this game via FFVII and Cloud.
Erdrick/Slime/Luminary: DQ got its start on the NES, and it is the grandfather of all JRPGs. DQ is getting an exclusive DQXI port with enhanced features, story lines, and quests. It's definitely showing itself already as a pillar of how Nintendo and Square have mended relations. For this current time period, Erdrick and his fellow reps here make a lot of sense.
Crono: Chrono Trigger is one of the most beloved retro JRPGs, more than likely, and it also got its start with Nintendo on the SNES. It was one of the last big Nintendo and Square collaborations (with CT being SNES-exclusive) before they made that split. Crono is a mark of how they'd be revisiting those old days when they weren't at each other's throats.
Finally, Geno: The most collaborative effort Nintendo and Square had before they split was easily SMRPG. It's Mario, Nintendo's most beloved flagship franchise, and Square's JRPG influence and creativity meeting together into a one-in-a-million effort. The uniqueness of their bond in olden days is shown through here, and Geno being the biggest name most people think of when thinking of SMRPG represents that.
Of all of these, Erdrick has the most in his favor in terms of third-party strengthening with Nintendo, with Geno coming second and Crono coming last. If this is their aim with DLC, I'd expect Erdrick, and if not a DQ rep, then Geno afterward. If Nintendo wants to go with multiple Square reps (with more than one
outside of Fighter's Pass, as I doubt they'd double dip), Erdrick in Fighter's Pass makes the most sense with Geno fitting outside of it, given he'd honestly just need to come with music. The Mario series has a lot of rep and stages, even an RPG-based stage (the Paper Mario one), so Geno honestly doesn't have much footing for Fighter's Pass. Erdrick easily is the best fit. If and only if they added more than one Square rep outside of the pass (as, say, misc. DLC or even a fighter-only season with no stages included), it'd give Geno a large second wind. However, for now, it's looking like Erdrick and DQ is what we're getting. I'm still expecting him, given Tansut has a flawless track record so far and the amount of evidence pointing his way.