The Fairy type, while...surprising (It was rumored/constantly suggested on 'type' wishlists for the longest of times, although I didn't think we actually needed it and that it would actually become real!) is now OK with me (I'd argue that it didn't re-type enough Pokemon - mainly for design/aesthetic purposes rather than balancing purposes), and I like how some of its type match-ups (mainly "Fairy vs Dragon" and "Fairy vs Steel) reference some tidbits of Fairy lore...
The only type I would like to see (and that makes sense to me) is Light, maybe Crystal (and that's mostly because of Crystal-style Jutsu that I read about on the Naruto wiki), more in an elemental sense (like how Digimon and Yu-Gi-Oh! "utilize" it, I guess). The Dark type has shifted from solely "underhanded tactics" to including actual "darkness" element manifestations (Dark Pulse says hi!), overlapping with the Ghost type and the "Shadow" type). The only thing I'm torn about is which moves and/or Pokemon should be re-typed (for instance, Luster Purge, Doom Desire/Flash Cannon, and Solar Beam, all of which add type variety with their respective Psychic, Steel, and Grass typings, mention and/or utilize "light" in some way). Light would also have to be made somewhat distinct from the Fire and Electric types (both of which also produce "Light").
While ice contains crystalline structures, I feel like "crystal" could easily be differentiated from "ice", since there are crystals that are nothing like water/ice...
Lava as an element makes sense, although you can easily fix that with dual-typed Fire/Ground, Fire/Rock, or even Fire/Steel moves in future generations, so I don't think it needs a whole type assigned to/dedicated to it. Maybe Magcargo could benefit if it were Lava (Fire+Rock)/something else, though..
Anything else like Sound (I like the mechanic of sound-based moves, just not the concept of a Sound type), Plasma, Space, Cyber, or anything else along those lines just sounds too forced for my tastes. And at this point, I think we have most of the "type/element/unique animal classification" bases covered..
The Shadow-type mechanic (roughly super-effective against EVERYTHING that's "non-Shadow" {XD}) introduced in Pokemon Colosseum has potential if it were used sparingly in the handheld main series games (where, on occasion, you would need to face and/or purify some "Shadow" Pokemon), and it also allowed better manifestations of "elemental Darkness" than the "Dark" (English) type does. It works nicely as a "type mechanic", although I don't think it needs to be a full-fledged type itself (although you really could consider it an "18th type" in Colosseum due to type matchups, I perceive "Shadow" as a type mechanic since you can outright lose the classification/condition with purification).
I personally consider the ???-type as an actual (albeit unused and un-assigned) type (all types are hit for regular 1x damage, and the ???-type lacks any weaknesses, resistances, or immunities); the one Pokemon I think that is deserving of such a type (if used in mainstream Pokemon gameplay) is Missingno., being the lovable-yet-scary, quirky glitch "Pokemon" that it is.. <3