We know that Marvel Ultimate Alliance 3: The Black Order is getting three DLC packs, the first of which is available now. Who else is coming in those other packs? That's what we're waiting to find out, but with the addition of the first DLC pack, we may have an idea as to how many characters are coming.
The addition of the Marvel Knights DLC, a new row of character icons has been added. That row includes 13 slots altogether. 4 of those slots are taken up by the Marvel Knights, and then after that comes X-Men and Fantastic 4. Seeing as how X-Men and Fantastic 4 DLC will most likely include 4 characters, that includes one separate character as DLC. Altogether, so long as the slots equal out to the DLC coming, that means we have 9 characters to look forward to.