Solomon Muto first appeared in the original series of Yu-Gi-Oh! He owns a game shop and he's the grandfather of Yugi Muto. Solomon Muto is the one who gave the Millennium Necklace to Yugi Muto. He also made several appearances in Yu-Gi-Oh! GX.
My guess would be Blue-Eyes White Dragon personally. There aren't many options that make sense. (I would say Exodia's the other likely candidate, but I don't think Konami wants 20 card Exodia to be a thing here, and I agree. Maybe they could use Legendary Exodia Incarnate, though, assuming they give him Grandpa's Cards or equivalent as a skill, which I expect they would.)
But Exodia IS already in the game, after all. Sure, as a 25 card and not an average 20 due to it being a skill thus needing a 20 base count anyway, but I could see a deck him using being Exodia and Exodia being his 'ace card' as it were. Though that being said that also wouldn't work as an animation couldn't play of him summoning it as it already has a victory condition animation...
Skill at stage 4:Too old to play
Makes your turn go second
Skill at stage 6:Heart of the cards
You draw at least one Super Rare card and start with 4500 LP
Skill at Stage 8:Grandpa’s cards
Starts with an Exodia care in the hand
Skill at stage 10:Puzzle Solver
Knows what cards are face Down and drawn by the opponent
Stage 14:Game shop
Starts with the field spell ‘Game shop’ that increases ATK points of all Spellcaster and Fairy types by 200 points.
And he received a new one for his lines in Bonds Beyond Time and Dark Side of Dimensions. I doubt either of Maddie's major characters (the other being GX's Sartorius) are out of the running. The only thing that makes me question Solomon is Yugi having an Exodia skill already.
Thanks for telling us this, I was wondering where he got the puzzle from even though I saw him give it to yugi. I was also wondering if ishizu really had the millennium necklace around her neck because maybe it wasn't.