I actually already have the original 72 masks, plus gold and silver, for a few years now. (I did a challenge where I built a model for every mask of the Toa's 36, which resulted in some cool Rahi and a lot of Meh Rahi.) One tempting idea is to go for the misprints... I currently have 5 of the 14, but gosh that's expensive. I also at one point had a nearly complete Nuva collection, but I never used those masks so I gave a bunch away. (And I don't regret it, those were ugly masks! Well, except for the silver ones... those sell for so much now adays that I do sorta regret handing a few of mine away, although I think one was a trade to Chocolate Frogs...)
Of course, my mask collection is almost entirely packed away in a container, which is kinda sad. So this recent burst of collector-itus spurred me to buy parts to create a stand so I can have my masks on display. That set me back a bit, but mostly because I'm insisting on using Mata heads with eye pieces.
Then I considered Krana. Like the Nuva masks, I handed away a bunch of random krana for a while. But I kept quite a few, and when I checked em out, I have about 1/3 of the original batch left (Both active and inactive), and nearly 1/2 of the Krana Kal. I'd be more interested in completing my original Krana batch, so I might be interested in buying and/or trading the Krana Kal? I dunno, haven't thought too seriously about this one.
I could do Kanoka, but there's not much collectability value in them, even if I'm a sucker for the Kanoka powers that the codes record. But that got me thinking about the original collectible... the disks from the Slizer/Throwbots! Each Slizer had 6 discs to their name, each with cool artwork, but only the first two were ever in sets. See, back then they had "disk packs" with random discs, but they were only available through limited retail (or maybe even just the Lego catalog) so it was harder to collect them compared to the eventual Bionicle Kanohi collections. But they still exist, and seem to be poorly valued on Bricklink... so there's an idea!
That said, if anybody has any krana or Slizer discs, I'd be willing to buy or trade something. (I still have three silver Nuva masks, plus a handful of others) (Also for the Slizer discs, not one of the ones that came in the sets with them throwing the discs and their elemental emblem... I have those already.)
As for the kraata... no, just no.