Type (re)Design 1: I doubt the logic of type differences will ever be rigorously explained in-game to satisfy veterans who grew up with the series but I am curious about tinkering with weaknesses/strengths from a design perspective. For instance, I think it’s a design flaw that Rock- and Ground-type – hardy “earthen types” if you will – have so many weaknesses in common because it that discourages these types from appearing together in future designs given how crippling a 4× weakness to both common Water- and Grass-type attacks can be. An early idea I had was to combine these types into a single Earth-type but I realize this is unlikely and would mean cancelling out some of the more interesting resistances/immunities of either Rock- and Ground-types. My other idea would be to remove Ground-type weakness to Water-type attacks (becoming 1× normal damage) and remove Rock-type weakness (also becoming 1× normal damage) to Grass-type attacks. That means the Rhyhorn and Geodude families for instance would only suffer 2× weakness to either Grass- or Water-type attacks.
Thoughts? Or is this plea overly specific?
I have… a bit of a history of badmouthing Ground as a type that doesn’t really have a point, or any thematic unity. You could get rid of it, I think, and we would manage without it. I tend to think that, all else being equal, a smaller and less complicated type chart is actually better, as long as it doesn’t restrict design space. There is an argument that we need some variable in the game that makes Flying Pokémon immune to Dig, Earthquake and Magnitude, but every other strength and weakness of the Ground type either overlaps with Rock (as you noted – weak to Grass and Water, strong against Fire) or doesn’t actually seem flavour-essential. Why do Ground attacks need to do extra damage to Poison-types? Or reduced damage to Bug-types? There are also plenty of Ground attacks that… don’t seem like Flying-types should automatically dodge them? Drill Run, Mud Shot, Sand Tomb, Earth Power… Bonemerang, for heaven’s sake. I think it would actually make more sense to have specific attacks flagged as “this doesn’t work on anything that flies or levitates.”
Pokémaniacal 
Well, let’s see…
Flying-types, like flying animals, must be understood to have lighter bones to facilitate their flight. Thus, it follows that they can easily outmaneuver a heavy, lumberous Kangaskhan bone when it’s thrown at them.
Feathers are often specialized to be hydrophobic, and mud is partially composed of water, which means that Flying-types easily deflect any mud thrown at them. Similarly, fine down protects Flying-type bodies from Sand Tomb (but not Sandstorm, because that’s Rock-type sand, not Ground-type sand – it’s all explainable with some deeper knowledge of quantum physics). For this reason, Pokémon like Tornadus, Gyarados and Minior are unaffected by these attacks.
Because the Earth’s Power reaches far lower than a lofty tower, and also the sky, Flying-types won’t cower – at any hour – to this upside-down tectonic shower.
As for Drill Run, well… in the beta version of the original Red and Green, the Flying-type was known as the Bird-type. As birds are, in this sense, the archetypal Flyings, one must conclude that birdly anatomy applies to all Flying-types. Birds have beaks, or: *bills*, if you may. And since “B” as in “bill” comes before “D” and “R” as in “Drill run” in the alphabet, any sufficiently organization-minded poet wouldn’t dare to use “bill” as the finishing rhyme – the sufficiently organization-minded poet would obviously use “bill” first, and finish with “drill”, in sentimental reverence to the originators of the Latin alphabet. As Satoshi Tajiri is in fact a direct descendant of these originators, it is only logical that Game Freak threw in this reference: Flying-types (especially Ducklett) always see a Drill Run coming, and act first – decisively.
As is plainly evident, all these immunities make perfect sense!
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