This dang hobby.
So here's what I'm missing from the units in the ASC rulebook-as-written:
-A space marine devastator squad (which, again in the RAW, has 3 bolters, a missile launcher, and a heavy bolter, always and only)
-A selection of genestealer hybrids
The hybrids are easy - a box of neophyte hybrids would give me a nice selection of weapons (and, not incidentally, a nice Shadow War: Armageddon kill team).
The marines are where Pandora's box opens.
The first option is to get a box of Mk IV marines (partly because I just really want to get some Mk IV armor). This would give me:
-A RAW devastator squad
-Marines left over to make a second 5-man tactical squad with a plasma gun, which I could paint as a veteran squad just for fun.
Nice!
Ah, but! If I also get a box of Devastators, I could not only mix up the armor marks and make them blend in with my current all Mk-VII force a bit better, but I could build:
-A devastator squad with a nice selection of heavy weapons (any combination of two heavy bolters, missile launchers, and heavy plasma guns)
-Marines left over to make a 5-man tactical squad* with an option of either a plasma gun or a missile launcher.
But but BUT! If I ALSO get a box of Mk III marines, I could do even more mixing and matching of marks, and also build:
-A devastator squad with a nice selection of heavy weapons (any combination of two heavy bolters, missile launchers, and heavy plasma guns)
-A full 10-man tactical squad, broken in to 5-man teams with either plasma or a missile launcher
-A 5-man tactical / veteran squad* with an option of either a plasma gun or a heavy bolter (because the above would use up all the missile launchers I'd get, since the Mk III box sadly doesn't get one.)
Each of these expanding choices ALSO gives me more and more actually-useful forces for full-on 40K as well, if 8th edition helps with a local resurgence of the game. (Squads with *'s could be fielded as 6-man squads in that game, too.)
And because mixing and matching marks is a goal, I'd want to get everything at once. Ah, me.
(Edit: Oh, I also don't have grots, but eh.
I'll pick up a bunch of 2nd edition starter box grots from ebay for 500 yen at some point.)