One suggestion I've seen a lot regarding a potential Bionicle return is the return of mask packs. I would normally be quick to dismiss such a notion given the utter failure of the original collectibles packs. But given some thought, and comparison with modern Lego lines, I think a similar product might actually have the potential for success.
The original Bionicle mask packs were a failure largely because they didn't offer much play potential on their own. A mask is just a mask (albeit usually with a head piece and axle to go with it). There is no play potential without an accompanying Bionicle set. Later series of Collectibles tried to fix this by offering ammunition for some sort of launcher, along with the launcher itself (to allow for independent play). But these were far less attractive than the masks, due to the fact that they were largely interchangeable. They lacked the distinctiveness of differently-shaped masks.
Lego has had more success with blind-packaged products since then with the Collectible Minifigures, which offer a clear advantage in that they are each a complete product. Even if you have nothing but a Minifigure (no other Lego), they are still a poseable figure with personality and charm.
So perhaps by including a figure of some sort, you could make a diverse range of collectibles a success! Matoran are the obvious option—while they were completely underpopulated in previous Bionicle waves, perhaps by randomizing them you could allow for as much diversity as shown in media like the Mata Nui Online Game.
Unfortunately, that introduces another issue. How many Matoran would people be willing to buy? If they're all the same build, just in different colors and with different masks, they'd be far less appealing than the diverse waves of Collectible Minifigures. And Matoran would be a building toy by any definition, which would prevent them, legally, from having any differences in part count, limiting the potential for diversity. The Minifigures, on the other hand, qualify only as figures, even though they require minor assembly. For those wondering, this is the same reason why individual Star Wars minifigures can't be sold—they would occupy the same legal definition as action figures, and the trademark for Star Wars action figures is held exclusively by Hasbro.
So something's got to give. The first option I considered was to include some sort of accessory. But that wouldn't be enough, I think, to make Matoran appealing despite being recolors—especially if they were randomized. Moreover, any substitute for mask packs would have to include at least 30 varieties: six shapes in five colors (omitting the six "natural" masks of your given Toa team). And to make distribution fair, all thirty-six would have to be included in a given shipment, which would be nigh impossible given that a Matoran-sized blindbag would inevitably occupy more space than a Minifigure bag. Then it hit me—what if randomization itself wasn't necessary?
Collectible Minifigures, after all, are not the only impulse-sized sets that could replace the mask packs. Enter the Mixels. This year, there are three waves of nine Mixels each—27 sets in all. Each one is distinct, having a unique appearance and personality. And they can do this because they're not blindbagged figures, but rather individually marked building sets. 27 sets is much closer to the 30 mask varieties you'd need. Perhaps the Mixels business model would be a better one than the CMFs.
So what I ended up with isn't mask packs, but the end result is similar. With three waves of 10 Matoran-sized figures, you could both create a full range of masks over the course of a year, and as a bonus, create a diverse population for your Toa to protect. Online, each Matoran could include a bio to help flesh out the world, and you wouldn't even have to guess to get the mask of your choice in the color of your choice.
The reason I'm not posting this in any of the existing topics about a Bionicle return is that it's both unlikely, and not fleshed out enough to be practical. Realistically, we'd see more than six new masks in a year—after all, we'd be likely to see villain sets as well, and in fact if the sets follow release patterns from the past nine years, the villain sets will likely be interspersed with the hero sets. The release schedule I've proposed is buggy as well—the second series of sets would end up having to include mask shapes and colors from the summer wave of "canister" sets before that wave is even released, which could easily harm the sales of those characters who were in a sense, "pre-empted" by small, impulse sets. I certainly don't see this happening if the theme comes back next year.
So why post it at all? Because I put a lot of work into brainstorming it. And because, even if only hypothetically, it shows that a diverse range of masks (and with it, a more robust variety of cheaply-priced Matoran) may be more practical than I might have previously considered.