Something to keep in mind about Fighter's Pass 1 is that apparently
it was in the negotiations phase by E3 2018. This is probably why it had so little basis in addressing fan demand, they likely just assumed people would be happy to see their favorite characters as spirits and assist trophies. For direct contrast, Smash 4's DLC wasn't a planned thing until it happened, which meant the first thing they did was address one of the games' biggest criticisms, "WHERE THE **** IS MEWTWO!?", even though Mewtwo was already a trophy. And even then, for a while
Mewtwo was the only character planned for DLC. With that in mind, it's worth thinking about that Fighter's Pass Volume 2 is gonna be their first opportunity to address a lot of roster-related criticisms in the vein of "This character should've been playable instead of a spirit/AT/whatever!" And while the ARMS character may not be one that was spirited/AT'd, it speaks volumes that they're going back to a series that was relegated to a side role in the base game.