I'm no expert, but it really depends on what you want your army to look like, and how competitive you want to be. Grots/squigs/troggoths are the main split.
Stabbas are good, but you really need to run them in big blobs, which I personally don't find much fun, though YMMV. What you are doing there is buffing up big units with extra attacks from Sporesplatta fanatics and sneaky snufflas, along with the loonboss' command ability.
Squigs are fun, though less competitive (though still viable if you aren't looking to place at tournaments). You're going to need squig herds for battleline, then a couple of big units of boingrot bounders, and then a couple mangler squigs. Maybe some squig hoppers for a different battleline and harassment and capturing objectives on flanks/in the backline.
Troggs are probably the least competitive, but are a small model count elite army. Troggboss, some units of 6 rockguts, and units of 3 Fellwater.
Of course you can also mix and match. Regardless of what way you go, I'd pick up a Fungoid Cave Shaman or two.
If I was basing an army off the battleforce, I'd probably get a shaman, a couple of boxes of stabbas, maybe a squig herd, and see how I go from there.