There are 10 remaining character spots on the official web-site. I read the FAQ page as saying that, if they add more characters between now and launch, those additional characters will not be included in your purchase if you pay the $200 for the Ultimate Pack:
"The Ultimate Pack is our best value Founders Pack, giving you access to nearly ALL COSTUMES AND HEROES in Marvel Heroes at launch, as well as four exclusive costumes. As 26 heroes had been announced before Founders Programs began, after the launch of Marvel Heroes, having an Ultimate Pack on your account will grant you all 26 announced heroes. It is possible that four of those heroes will be post-launch heroes, specifically Luke Cage, Emma Frost, Squirrel Girl and Nova. These Heroes will be added to your account automatically as they are released. Beyond this point, when future heroes and costumes are added to Marvel Heroes, they will be available to you as an optional purchase, or as a random drop in-game." [emphasis added]
I don't know if they are going to add more characters between now and launch but, if they do, it seems stingy to not let the people that buy the very expensive Ultimate pack not have those characters too. Personally, I would want Invisible Woman on my playable roster...I would be very upset if she appeared b/f launch as a playable character and I had paid the $200 only to find I now have to pay more for her.
Edit: The promo art for the Ultimate pack reads ALL characters and costumes available at launch. There is clearly a difference in the FAQ page that warrants some clarification from the producers.