I don't think they'll ever make a starter that becomes part Ghost. Or Bug.
Hm...now that I think about it, I don't think ANY Pokemon ever suddenly acquires those types through evolution except Shedinja. So that's probably why. If something is a Ghost or a Bug it pretty much starts out that way and gains something else or loses it.
And I dunno, perhaps they WOULD make a Dragon after a while and are afraid of looking like they're running out of ideas. Probably with the Grass starter if we still don't have a Grass/Dragon. Or if we still don't have a Fire/Dragon besides Reshiram they could do it with the Fire starter.
It's technically possible. I see it as more likely than Ghost or Bug.
By the way, think they'd ever do Ice? The Water starter would make the most sense though I'm afraid.