Ho-Oh's Weakness is the best selling point, imo! It's incredibly valuable to have diverse Weaknesses in a mono-type deck, which in particular Volcanion has to be by default, given Steam Up's prerequisites. In fact, along with Talonflames PHF and XY, I think Ho-Oh-GX is among the only Fire-type Pokémon in Expanded (let alone Standard!) to have a Weakness other than Water. Perhaps it isn't the best inclusion for a Pidgeot deck, but it's certainly good for Volcanion heading into Worlds and beyond. ^.^
If you're looking for an easily splashable tech for Tapu Koko-GX in Pidgeot (which I wasn't aware Tapu Koko-GX was played all too often, but hey), might I suggest Lycanroc-GX? It two-shots Koko and one-shots most other Lightning-types, and comes with the added benefit of Bright Look.
If it's specifically birds you're looking for, I'm not sure I can help you there––Flying-types tend to just have Lightning Weakness in the TCG. I wouldn't say Ho-Oh "messed it up", though; the alternate Weakness is something I think the Fire type really needed.