Arle is my most want since Melee alongside Ayumi.
I think now she is definitely the top contender for the next set of DLC.
However, her biggest issue is still the copyrights problem. If I understand correctly, the rights for Madou Monogatari (so her name Nadja, her old design, her ability to fight in general, and her spells) belong to D4Enterprise, the rights to sale Madou Monogatari belong to Compile Heart, and the rights for Puyopuyo belong to Sega.
I personally prefer Arle to represent a good amount of Madou Monogatari elements, but to make herself as a fighter in a fighting game, I believe D4E has to be on board as well.
I said it's the biggest issue for Arle, but now they somehow managed to get Dragon Quest of all thing in smash so I'm very hopeful for her future. The problem still remains but it's not a huge blocking wall than used to be.
Alter proved that Sega and D4E can work together to bring back old Arle(ish) design. I'm hoping her smash render to be like this.
https://alter-web.jp/products/222/
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Also, Sega is treating her as one of their biggest mascot (and of course they name Puyopuyo before Sonic!), she has to make into smash at this point.