Plus, given his horns, he's potentially part Ancient Giant. Or...something else? Either way, he might have some sort of important mixed lineage.
And there's a non-zero chance that his Shadow Fruit powers are somehow connected to the Abyss Mark system, somehow. Not because he has a mark. But, because the Shadow Fruit could be tapping into the same "power source" that the Abyss taps into. Or possibly sharing some elemental connection to Blackbeard's Darkness Fruit. All these "dark powers" could be connected in a interesting or important way.
And, his Shadow Fruit's power to revive the dead could be VERY handy if they find some legendary person's body along the way. Like, say they find the corpse of Nika or Joyboy or Davy Jones and have Moria revive them with his Shadow powers. Or even the Galle-Ra Giants. That could get kind of crazy. I'm not sure if that'll ever happen, because that seems too important to leave to "side characters". But, it could be useful in some other way.
Plus, while Thriller Bark itself is probably off the table, it's worth remembering that Moria did somehow turn a island into a mobile fortress. Which means....Moria is actually some kind of engineering genius, apparently? Or he knows someone who is. Either way, if he can bring that talent to Cross Guild, that means they have a "shipwright" whose talent would rival or surpass Franky's. Which is really handy if you're trying to maintain a fleet of ships. Or trying to build something that rivals Ancient Weapons.
Realistically if the Horo Horo no mi can't be resisted with haki there’s nothing that stops Cross Guild from soloing every faction
Interesting point. Because, in theory, Perona could be the ultimate support fighter in this series. Perona's Horo Horo Fruit is basically Anti-Haki. In that, if she can use her powers on someone, they basically lose all their will and confidence. Haki is based almost totally on willpower and confidence. So, again, in theory, if Perona can hit someone with her power, that would leave them totally open to a follow-up attack from someone else who's much stronger than her. So, basically, if Perona can work with someone else who's stronger, like a Mihawk or even a Crocodile, Cross Guild could actually defeat a lot of the Haki Monsters in this world. Like a Kaido or even a Shanks. Especially if they could manage to take them by surprise.
Mind you, it probably doesn't ACTUALLY work like that. Someone with enough Haki probably COULD push through Perona's powers. Because Haki's just overpowered like that. But, in theory, Perona should be able to be a big asset in a fight. Not as a main fighter, but partnered up with someone, that team should be something to be reckoned with.
So, honestly, Moria and Perona could actually add a LOT of value to Cross Guild. It's not just reading the Poneglyphs and a potential for a Zombie army. They could do a lot more than that.
But, the problem is, most of their value is purely hypothetical. We don't actually know for sure if Moria can read Poneglyphs, or if Perona's powers could work on strong Haki users. But, the potential is there, I feel. They could play a pretty big part with Cross Guild, if they ever join.