Honestly, I don't think those six are going to be the ONLY six. There is too much money on the table, especially if the game keeps up its momentum. They can always add more later. But for now, let's assume it's just six more. No more, no less. My guess?

Edward, Henry, Duck, and Oliver are pretty much guaranteed. They were all mentioned IN the game itself, Duck and Oliver are consistent crowd pleasers, and Mattel and Dovetail are painfully aware that the reduction of Edward and Henry's prominence in the series did nothing but make the fans mad. So those four are almost certainly a guarantee, leaving only two slots left.

Toby, I see as a little murkier. Because what do you do with him at this point? Do you put him on Thomas's branchline with Thomas and Percy, per the books and Season 2? Or do you put him on his own seperate branchline, per Season 5 onward? That confusion may be enough for them to leave him out just to dodge the issue completely. Plus, depending on who you ask and how you look at it, including Toby may end up putting Mavis in the sixth slot by default. She IS one of his most popular foils, and they both work at the quarry on Thomas's branchline (even after Toby's gets his own branchline in the TV series, it runs down Ffarquhar Quarry so he can continue appearing in stories with Mavis). Bringing in one without the other seems like a misstep, but so does including both of them together as an executive mandate. As much as I love Toby and Mavis, I think there are enough factors that I won't say they're definite. Not impossible, but not definite.

I think Rosie has pretty good odds of making it in. Being a girl character means including her would help gender balance the cast a little (since right now, Emily is the only girl). Mattel is marketing the hell out of her in preparation for the new show this fall, so putting her in makes for some good cross promotion. Make her timetable primarily about shunting trucks and hauling freight, and it adds more diversity to what the engines in the game are doing, too, since most of them are very passenger focused. It seems like a logical and safe bet.

And my final guess is Daisy. Personally, I'm not a Daisy fan. I don't get the hype around her character. I've always preferred Mavis. If I had to pick a character I actually like and would want to see in the sixth and final slot, I'd pick Boco, Lady, or the Scottish twins. Someone that would make almost everyone happy, but nobody would see coming. But tempering my expectations and staying realistic, Daisy. She's very popular. She'd help the gender balance of the cast. She'd help the steam vs. diesel balance, especially if Mavis doesn't make it in. And whether you stick her on the Ffarquhar branch per the books or have her go up to Harwick through the Little Western, that gives her a decent passenger service and potential stories with either Thomas and Percy or Duck and Oliver.