Hmm... has it ever been officially established that the naming order was planned? It would seem a bit much of a coincidence, though, if it means we might see another 20-odd Alien movies while they fill in the rest of the alphabet, I’m in!
In the meantime, I did some name research and came up with some interesting stuff, especially for Walter. I’m sure none of this was seriously considered when choosing the character names, but I thought it was interesting after-the-fact:
ASH: Short form of ASHLEY (from an English surname originally derived from place names meaning "ash tree clearing"). It can also come directly from the English word denoting either the tree or the residue of fire. Not much here, except maybe in “residue of fire”. ;)
BISHOP: Either from the English occupational surname, or else directly from the English word. It is ultimately derived from Greek for "overseer". He had no real authority as an overseer, but he seemed at time an ethical center for the plot (“harm or allow to be harmed...”) and was the go-to guy for some needs. Of course, he did some pretty intense “overseeing” of that facehugger carcass.
CALL: Irish; reduced form of McCall. English; from Middle English calle ‘close-fitting cap for women’ (from Old French cale), probably applied as a metonymic occupational name. Not much here, except for “close-fitting” which sort of describes her personality.
DAVID: From the Hebrew name meaning "beloved". David was the second and greatest of the kings of Israel, ruling in the 10th century BC. Weyland prized him above others, but not sure it was love.
WALTER: From a Germanic name meaning "ruler of the army". Could this foreshadow something he does with the knowledge he ‘steals’ from the Engineers?
At any rate, a fun little riff on your question. Here's hoping we'll have more to ponder with the release of the film and book?
