HND is opening 6 slots up to US carriers in 2020 and I'm confident American and Delta (and maybe Hawaiian) will apply for them, too. If it's like the last round, United will get 1 or 2.
Isn’t a slot one take-off or landing? Getting one slot would mean parking overnight. Might 6 slots really mean 3 planes in/out? I’m not sure about Haneda, but overnight parking is scarce at most Japan airports.