I have been painting Warhammer minis for years, assembling the equivalent of a 1500 point army of Space Marines with a few Sisters and have played 3 times since 2008. The hobby is more interesting and less daunting than the collecting and playing of a full fledged army. Kill Team is the exception and I am excited to be able to jump in and play with a few of the pieces that I already have (and a few new Intercessors). I am only upset that I can't bring my Librarian or Battle Sisters into the fray...
I am reasonably sure there will be future support for sisters of battle, sisters of silence, chaos demons, the Inquisition, and other factions not in the core book. Perhaps a supplement book or starter kit. Perhaps White Dwarf articles.