rg, first I plan to finish my Bionicle retelling, and then hop into working on the Season 1 finale. No specific schedule for that, but the Shards polls will have to go up before I get too far into it since I plan to feature some of the winners in it.
SK, well, crystal is a very different feel of a substance from other things, like its closest relative rock, and it's never really been clear in official story whether it counts under rock (not that that would necessarily matter for EM), plus it's one of the all-time most popular candidates for an element among fans, and the EM tries to appeal to the fanbase where plausible. Crystal was even at one point one of the top pollers for a suggested addition to official elements. Plus, despite never being stated to be part of any known official element, it is a very common thing in canon Bionicle, especially lightstones.
Stone has microcrystals but Crystal in EM means crystallization over that point.
Tar is also a fairly different substance. As far as I recall it's the only clear "gooey" element in its natural state -- somewhere between solid and liquid, and it can occur naturally.
Sand is already a different element canonically (SM).
Gold and Silver are separated out primarily due to the world of Izumal in which they are the two most common (land) elements, and both are used extensively in the city (Gold for underlying structure and silver for nanites). Silver is also used as the special Toa Tool substance. Gold is an especially malleable version of protometal so has a different range of uses than for example the metals that are more like Iron, plus it is precious so it makes sense to have it be separate.
Also, Greg specifically confirmed that having gold and/or silver or the like as separate elements would be plausible even for canon, had there been any reason to have more room for a larger elements list (and that's basically the idea of Expanded Multiverse elements, to be expansive, along the theme of the name).
I don't know why you bring up salt; I don't recall what we did with that and have no time to check at moment.
Ghost refers to intangible energy structures. Given that such energy effects are actually very common in Bionicle and indeed much fiction, and that it's a unique and interesting substance with story potential it deserved its own element, we felt.
Lava and Sulfur are also both EM-world-based elements (Tanuuk and Promathus), plus are also distinct materials. Lava is the most obvious -- official story actually separates out water and ice, so liquid rock (deadly hot) and cold rock are separated on the same principle (that plus Earth, Sand, and Stone being different also especially founds the whole idea of a fuller expanded list; the only reason the canon does not is that it had no room to feature them, but we clearly do). It's also a unique and very important, and very Bionicle, common natural substance. Sulfur is natural though not so common in Bionicle or real life but also has distinct poisonous traits and was used as the theme for the industrial-pollution world.
Fungus is actually the third major branch of life, similar to plants but not, so is one of the clearest ones that gets its own.
Virus is similar but inherently microscopic.
Mercury again has very distinct natural traits (at room temperature) and had lots of story potential (especially Toa Merx), and was another that Greg agreed could make a plausible separate element if a story had room for it.
Alchemy is possible in Bionicle, interestingly, since it's all protodermis (in MU, and more to the point in the EM), but is that not making gold? But it would be difficult as you'd have to "hack the base code" of protodermis particles. Or you could be a Toa of Gold.
Occultism is definitely something we shy away from! I don't know how you feel that would be okay but many people consider it evil and not to be toyed around with in kid-friendly fiction. Some say HP does this and/or amounts to an exception but the point is that's not at all Bionicle's style nor the style we're going for. This story is, however, inherently mysterious and could have some similar traits to this but what that would have to do with how expansive the elements list is, I don't quite follow. Actually there is an underlying reason connected to deeper mysteries here why the list got expanded.
I'm getting the sense your preference for the old fan theory that Toa had wider ranges of control may be motivating your approach here. But this is not really our preference nor would it seem to be the preference of most fans. But symbolic attachments to these elements are certainly fair game. I do that with some things in my Paracosmos fan fics, for example. If you want to delve into that kind of thing, that's certainly fine! We were going more for the "elemental feel" and effects and personality connections and the like.
Also, it seems to me that the old interpretation of Toa as demigods that had wide ranges of control was never consistent to begin with because of the separation of ice and water, or at least that if you feel even that narrow separation can work with that feel, why not the very similar but more numerous ones we've done here too?