I think the ability of most Legendary Pokemon might be plain abilities, not HAs
So that means, we might not get Multiscale Lugia, Regenerator Ho-oh, Telepathy Dialga/Palkia, Defiant Tornadus/Thundurus, Volt Absorb Raikou, Flash Fire Entei, and Flame Body Heatran if that happens.
We have already a TCG Product related to Raikou and Entei this April. Good hint.
Suicune was the Star of the Pokemon Crystal. So it makes it clear as a third-party Pokemon. Plus, in USUM, you need both Entei and Raikou to call Suicune in Wormholes.
So, since the trailer says New TCG-related Products every month related to Legendary Pokémon, I assume these products are:
Dialga-GX/Palkia-GX Box
Regigigas/Heatran GX Box
Entei/Raikou Box (makes sense! Incoming box in April)
Xerneas/Yveltal GX Box
Shiny Zygarde GX Box
... And so on.
I'm not sure though