Here is a more accurate picture of unused type combinations, factoring in primary vs secondary types as well:
Fully visible: no existing pokemon of this combination at all
Quite visible: certain pokemon gain them temporary through formes, or pokemon that are of this combo have their type changed upon evolution, meaning there is still no fully evolved pokemon with the combination
Half visible: there are pokemon with the reverse combination, but none that are exactly this one
Almost invisible: one or more pokemon of this type already exist
1. How is this still not a thing
-Bug/Dark
Like, seriously. We have 6 evolutionary lines of Bug/Steel, but not a single Bug/Dark one? This combination feels as natural and obvious as Grass/Poison.
-Fire/Poison
Another one where you'd think "I bet there is at least one already", but there just isn't. We actually already have a candidate for this in Littens evolution, but I wouldn't bet too much on it.
2. fair(l)y interesting
-Fairy+Electric/Fire/Ice
Now that we have the basic fairy types established (a /Flying one, some basic Psy and Grass combos), we could use some elemental combos to spice it up, but with Fairy as the primary type (so not like Dedenne where Fairy just feels like an afterthought)
3. WOW SO COOL
-Electric+Psychic
especially if it made use of the cliche colours, combining purple and yellow in its design
4. just do it already
-Bug/Water
if nothing else, at least we need this so a NFE Pokemon isn't the only one with that combination, but there is lots of insects that live in and on water anyway.
A very unexpected turn would be if this combo was used for a pseudo-legendary (to get away from all the Dragons, why not a Bug-type instead, coupled with a decent defensive type), based on giant isopods perhaps?
An additional question worth asking in this thread is:
What type combinations do you want to see more of?
Of couse just because a combination has been used once, doesn't necessarily mean that that particular combo is now covered in a satisfactory manner.
For example, yes we have Lanturn as a
Water/Electric, but there is still a lot of unused potential with that type that hasn't really been touched since gen2.
Another example I can think of immedately is
Grass/Ground. Torterra is the best design you might want from that combo, but you'd expect this to be used more often than Water/Ground (which also has like at least 5 lines already IIRC), or is that just me? xD
Similarly, yes we have Reshiram and Charizards fanservice Mega evolution combining
Dragon and Fire, but it is kind of surprising we do not have a regular evolutionary line with that combination yet.
One more thing I want to mention quick is
Flying as a primary type.
It's the only type that has an absurdly high "used as secondary" ratio, something like 99%.
Although we did get Tornadus (still the only pure Flying) and the Noibat-line recently, Tornadus hardly counts due to it being part of a trio that shares a design-basis (meaning its design doesn't give much insight into what it takes to be a primary Flying type), and Noibat doesn't seem like a simple Pokemon that should be used as a clear example of what Flying types look like. So what I'd like to see is, a 3-stage pure Flying-type Pokemon, or that there would be at least 2 more lines with Flying as their primary added ASAP.
EDIT:
oh lol, I've been typing this post so long, His Goomines covered some of the combos I'm talking about already XD
And oh yeah Electric+Steel is probably an even better example than Water/Electric for already used combos we need more of.
