10. Odo
The last we saw of Odo was him descending into the Great Link on the Founder's homeworld. Now given the importance that they all saw in Odo, it could be expected that he would eventually become what is essentially the Dominion Ambassador to the Federation. After all, he would be the only Founder to have the required balance between what the "solids" want and what is in the best interests of his people. Odo is also probably the easiest character to age out of those that previously wore a great deal of make up. After all, what if the look that the character was trying to achieve all along was exactly how René Auberjonois looks now? Odo has been systematically linked to what could be millions of other Founders, all of which have mastered the imitation of solids a great deal better than he did during the run of Deep Space Nine (barring the Cardassian neck trick, of course). So in order to have Odo appear, I'd simply have Auberjonois in the series without his previous level of make up as the character. This would also have the effect that he could show a greater range of emotion, which would be another example of the level of maturity that Odo has now reached over the time between the end of Deep Space Nine and the new show.