Possibly I had forgotten that Phantump/Trevenant was also grass type, but no main character has truly trained a ghost Pokémon in all of this time and I don't necessarily see that changing now. Just as no main character has had a psychic type since Misty (and Starmie barely counted honestly) and no main character has captured a steel type during their tenure in the group (Brock didn't get Steelix until after he had left) and to date no trainer has caught a fairy type except Dedenne who isn't really a battler and has really been more showcased as electric though I'd like to see Serena change that, either with Eevee evolving into Sylveon or capturing something fairy type, I would think Spritzee. On top of that, if they want to give Ash a ghost type I truly think that Honedge/Doublade/Aegislash (yes I want that full line) would be better, his first steel and his first ghost type.
Haunter way back in the first season doesn't count as a ghost capture because Ash didn't truly capture it.
Skiddo/Gogoat on the other hand is just grass and doesn't face that little quandary. Also for what it's worth, when Ash saw Alexis's Gogoat he did say he wanted one though of course that means very little.
That's not entirely true. No main protagonist has ever trained a Ghost-type, but Jessie and James count as main characters, do they not? James had Yamask, and Jessie has Pumpkaboo. Same with Psychic-types- Jessie had Woobat, and James has Inkay. And you're outright wrong about the Steel-types, I'm afraid- remember Forretress? The lack of Fairy-types is more forgivable, given the type's relative newness. Though to be fair, before the Fairy type was a thing, Misty had Togetic and Azurill, Dawn had Togekiss, Tracey had Marill, and James had Mime Jr.
As for Serena's Eevee, I'm actually in favor of it evolving into Umbreon. It was shown playing underneath the moon at night, and while that could be hinting at Sylveon, it's just as possible it was hinting at Umbreon. Plus, if it becomes Umbreon, then that'll allow her Pancham to evolve into Pangoro, and I think there's definite potential there, in having Serena and Pangoro having to work on a whole new approach to Showcases to compensate for its larger size. I did always want Serena to catch a Swirlix, personally, but due to two separate rivals owning Slurpuff, I'm forced to admit that that may just not be happening.
And finally, it's true that no main protagonist has owned a Ghost-type (unless you count Haunter, which few people do), but there's a first time for everything. For the longest time, Dragon-types were off-limits for main characters, and then suddenly Ash gets Gible, Goodra, and now a Noibat, to say nothing of Iris. And for a long time, Dark-types were off-limits for main characters, and then Ash gets Scraggy and Krookodile. Maybe this is when he finally gets himself a Ghost-type. And I'm not so sure about the Honedge line. It's very cool, no question, but I'm just not sure I could see Ash with one of those.