d20 Call of Cthulhu is, I think, the most exciting development in the system. COC is such a different genre from D&D, with such different requirements, that it will really stretch the system and see just how flexible it can be made to be.
That said, I'm not entirely sure what use there will be for d20 Modern after COC is out, as it seems to me that there will be an awful lot of overlap. The only reason I can think of for it is to develop other settings, or retain a Modern ruleset IP -- as I recall, only Chaosium can continue development of COC after the initial WOTC release.