Agreed with Auty here. I might be one of the few people who actually enjoyed the Boom series, but that said I think the games ruined what Boom could have been.
I could see there being several different Eggmans/Robotniks, some new, some old...and different interpretations of mainstream characters.
But what I HOPE to see are...
Blaze the cat...I think that's a given.
She's already from another dimension, so I think just her existence should fit. (I was also hoping she'd appear at some point in Boom and it never happened...had the series continued though perhaps we would have seen everyone's favorite fire cat)
Some call backs to older cartoons like SatAM, AoSTH, Underground, and perhaps Sonic X and Sonic Boom. Perhaps get some closure finally on the endings to some of those series. (I always kinda wanted to see Knuckles appear in SatAM and AoSTH but you know)
Perhaps references to some of the other games canon. Technically, wouldn't Sonic and the Black Knight count as an AU? All the Sonic characters existed as Arthurian Knights, which was a cool concept (though I haven't played it myself...it just sounds cool to me.)
I think having a reference to the games and the phantom ruby would be a good idea...maybe the phantom ruby contributed to the shattered universe...
What we probably won't need is brand new characters....buuuuuut at the same time if they're good characters and add something unique to the plot...and are actually enjoyable...I wouldn't mind them too much. (I'll never say no to more cats though
)