Mokuba is honestly a terrible choice for a new character. What would make his deck or skills different from Kaiba?
Much better choices would be Bonz, Espa Roba(though Jinzo is OP), PaniK, Umbra/Lumis, or even Jaden from GX.
Actually, they'll probably just give him a similar deck to Kaiba's like they did in Duel Terminal. I do agree that Esper Roba would make an excellent addition to the game though.
If mokuba had a deck i would make it a strong trap and spell card deck and cards 2 increase his lifepoints and a few good monsters
Traps like mirror force negate attack
Shadow spell kunai with chain spellbinding circle.....shadow ghoul and discard 35 monsters and power up shadow ghoul and win the duel in 2 turns....