The current types cover everything very nicely. Nothing essential is missing.
There is the elemental types:
-Fire,Water,Electric,Ice,Grass,Ground
The species types (because they directly related to the organism itself):
-Bug,Ghost,Dragon,Rock,Steel
And the technique/strategy types:
-Fighting,Psychic,Poison,Flying,Dark
And Normal which is the non specialized/default type.
The usual ideas that people bring up include:
Light - This is completely covered by the energy types Fire and Electric. Mostly everyone argues that Dark needs an opposite type, but that is based on misunderstanding of the concept behind the Dark type.
Dark is not actually about darkness and shadows. Those are secondary effects of it and even more prominent in the Ghost type, just as light is a secondary effect of Fire/Electric. Dark is really a battling style/approach. Just as Fighting uses martial arts and muscles as its advantage, or Psychic using its superior mental abilities instead. Dark is the unfair strategy. Cheap, nasty, sneaky tricks. Simply biting, Flinging stuff, stealing, Sucker Punchs, surprise attacks..
In case that isn't obvious enough, note that the direct translation of the Dark type from japanese games is Evil type.
Wind - its a general extra effect. Usually whenever you have some windy noteworthy attack, the stuff it is tossing around is more important type-wise than the wind itself, ie Sandstorm, Leaf Storm/Leaf Tornado, Fire Spin, Twister. And when it doesn't, Flying has proven to step in and claim it, which is good as Flying is the least interesting type otherwise of them all. I wasn't 100% sure about that until Tornadus finally made it official.
Sound - Another secondary attribute that doesn't allow enough variety for a type of its own. You can cause sound with literally anything.
Sand, Mud or whatever - completely covered by Ground
Space - One suggestion I never really understood.. what would it be about, vacuum?? That's less interesting than anything else.
If the point was to add something more mythical or ancient, Dragon, Psychic, Ghost and Rock are the types to use.
With my personal three sided classification above, ideas that could come up would include more "species" types... maybe Mammal, Monster, Fish?
But those would just cause a giant mess... there's egg-groups to do that.
The question then though is, are the current species types necessary?
-Bug - the sole idea for the franchise came from the hobby of collecting insects.
-Dragon - mythological creatures that are an enigma, since there is nothing like them actually in the real world, yet every continent knew what dragons are since ancient times. It gets fishy when you get to Dragon-type attacks, but otherwise deserved spot.
-Ghost - since there is a default type, an Abnormal type makes sense. Its about weird creatures that dont seem to make sense biologically. Has a strong relation to shadows/night-time too.
-Rock - Mineral based creatures are quite extraordinary too. And it can be used as armor etc. Lots of unique aspects.
-Steel - in my view, a superior Rock type, but unlike rock it is about materials not usually found in nature, which is weird enough.
These don't imply there should be types like Fish..
So, the current setup is perfect.
Some of the types might not be completely honest,
(for example Ice is not about water ice, but about cold, or Fire which could be called Heat type to make it more general, or Steel changing to Metal or Alloy, Bug to Insect, and how Ghost types are not literally ghosts, so something like Abnormal or Weird would be more appropriate, Rock to Mineral etc..) but they are more simple words that anyone can immediately relate.
However there are some holes in the balance, so in my opinion gamefreak should do some more tweaks to the type chart. My full suggestions on that matter:
https://saiph-charon.deviantart.com/art/More-balance-204921036