If they completely ignore everyone in the base game (including those with spirits, seeing as they currently don't have any other artwork to use), it would make sense to go for anyone that matters more in the lore (Max Brass or Dr. Coyle).
But, then again, their other divisions make some weird decisions too. In some ways, Nintendo is equally as unpredictable as Mass of Heroes Samurai himself.
On that argument, a series has never been represented by a villain first before so, coincidentally, that basically rules out 3 of the 5 DLC characters anyways (Springtron, Max Brass, and Dr. Coyle). Going further with that, Springtron is basically "Dark Springman" and Dr. Coyle's gimmick is that she levitates a significant distance off the ground.
On the topic of gimmicks, here's what they are for each of the 15 characters:
-Springman: gets an attack boost with low health.
-Ribbongirl: double jump.
-Master Mummy: can heal while blocking, doesn't flinch against most attacks.
-Ninjara: can "smoke dash" in the air or on the ground.
-Mechanica: can hover.
-Twintelle: can technically briefly freeze time.
-Min-Min: attacks with grounded and aerial kicks.
-Byte and Barq: Robot dog assistant.
-Kid Cobra: charged dashes.
-Helix: basically can change shape and height at will.
-Max Brass: charge that prevents flinching plus Springman's ability.
-Lola Pop: DeviantArt
-Misango: spirit companion that boosts stats.
-Springtron: Springman's plus a shockwave attack.
-Dr. Coyle: can hover, can summon a third arm, and can go invisible if stunned or knocked down.
Going off of this, it further makes cases for the 5 that are already spirits because they have the most "Smash friendly" gimmicks of the cast aside from Kid Cobra.