[Deck Knight]

I'm rallying around Ghost.

Flying/Ghost does indeed have a lot of weaknesses, however it possesses 3 immunities, and is the only type combination that will substantially alter momentum in favor of Stall Teams by acting as a spin blocker.

Consider the following strong points of a Flying/Ghost combination:

Three Immunities:

It goes without saying that an immunity to Rapid Spin and Fighting Attacks generates a lot of momentum if you can lock an opponent into those attacks, which in all likelihood will be an option for a concept based on Momentum. Taking 0 damage when switching into two very common attack types and a third attack typing that is becoming increasingly more common is the best thing a Pokemon seeking to change match momentum can hope for.

Has vulnerability most often to only one coverage attack on a given opponent.

Rock, Ghost, and Dark attacks are fairly common coverage attacks, but most Pokemon only carry one of them. Excadrill has been running Return more and more and while Rock Slide is a threat, there are multiple ways to deal with a quick Rock Slide using a combination of Stats and Ability. There's also Shadow Claw which sees even less use. Conkeldurr may be the exception as it can possess Stone Edge and Payback, but Payback no longer doubles damage on the switch-in, so its less of a sure thing. Moreover, Conkeldurr is more likely to use Mach Punch to finish a threat or Drain Punch when it it low on health to keep tanking, and these are ideal opportunities that only Ghost/Flying can exploit to turn a game around. A resisted Conkeldurr Mach Punch or Drain Punch still hurts, and Flying by itself ensures Stone Edge hits for neutral damage at worst.

Hits the vast majority of Steel-types for Neutral damage.

We do not want CAP1 to be a sweeper necessarily, however having neutral coverage against common metagame threats is very important. Flying has neutral coverage against Ferrothorn, Scizor, Forretress, Excavalier, Durant, and Lucario. Ghost adds Metagross, Jirachi, and Bronzong to that list, and unlike Flying/Fighting, doesn't open up a weakness to Psychic.

Threatens Reuniclus and Conkeldurr simultaneously.

While both threats have coverage attacks they can utilize against Ghost/Flying, The typing hits both of them super-effectively with STAB attacks. With Psychic and Fighting both rising to prominence, being able to hit both while being weak to neither is a huge benefit. Psychic itself is experiencing somewhat of a Renaissance, and Ghost can strike Latias, Latios, Espeon, and Sigilyph effectively as well.

Remains invulnerable to Fighting during Roost

Yes, I know Movepool is not decided and will not be for some time, however this is possibly Ghost/Flying's greatest situational advantage. It works perfectly with the Imperviousness trait by ridding Flying of its 3 troublesome weaknesses while only opening it up to assault from one dangerous attack type (Ground) instead of two (Ground and Fighting). Dark and Ghost are common coverage moves, but present nowhere near the general threat level that Psychic (Flying/Fighting) and Fire (Flying/Steel) open up.

As far as the CAP Process goes, having a lot of weaknesses but some great core strengths opens up a lot more leeway for stats and abilities. Ghost/Flying is a type that exemplifies high-risk, high-reward and forces opponents to do specific things to counter it. Moreover many threats cannot realistically be handled even with nuetrality. Fighting/Flying may be nuetral to Landorus Stone Edge, but as mentioned earlier Landorus's Earthquakes can break through resistances. Nuetralities are not enough, and with a main typing of Flying, Rock attacks will always be a problem. Finally, Ghost/Flying's unique advantages lend itself well to using movepool to maximize its typing quirks. It will lure in a lot of threats that believe thier coverage move can handle it, and they will be in for a nasty surprise when CAP1 is behind a Sub. Sub + Encore for example is very potent when you have 3 immunities to work with, and Pokemon with 5 types of attacks think themselves threatening enough to deal with you.

Going back a bit to point two, since Ghost/Flying is usually only vulnerable to one coverage attack, you can tell almost instantly what an opponent's set is depending on how it tries to handle CAP 1. Combined with Disable (or Encore as mentioned above) it can force yet another switch and deal more residual damage to an opponent. tl;dr, Ghost/Flying's weaknesses lure in and help identify many threats from behind a Sub, and its strengths allow it the maximum number of chances to come in for itself.