You're right. It very well could just be a scythe (maybe he's a Botanist?)

I think there are quite a few hurdles in the way of making Zenos an effective villain for this next expansion. He's not particularly charismatic (Emet) or hateable (Ilberd). He's not cunning enough to be a schemer, and not unpredictable enough to be convincingly insane. He's also just not intimidating. We've beaten him enough times in recurring boss fights that he's almost the equivalent of Seifer or the Turks in this game. More midboss than final boss. Shadowbringers was a bit essential in this regard to give us a break from his character and give him time to do some training montages and practice his Sharingan- ...er, I mean Resonant techniques. But he needs more than that if he wants to enter the ring against the Convocation's maverick, partially powered up or not. Gaining Garlean-engineered fake Ascian status isn't going to cut it. Otherwise, what's the motivation in going to the moon to get a McGuffin to beat him with?

He also needs to make this fight personal. A lot of the really good Final Fantasy villains usually do something to properly incur your wrath before they go about ascending into godhood and creating a World of Ruin. Again, vandalizing property with fireballs doesn't really cut it.

That's why I think that he just needs a bit of guidance, from some of the better villains that we've encountered in the story thus far. Nothing like having Mephistopheles sitting on your shoulder goading you to further heights of badness, and a Reaper voidsent Avatar would do just the trick. It would also make for a great council fight concept, and after being pleasantly surprised with how slickly they pulled off the Menenius/Lyon tag team in Bozja, I'd really like to see these team based match-ups as the baseline rather than the exception in actual raids. You could even throw in a Possession meter and have the two merge to burst down a single tank or split to cause an enmity reset. You could do some pretty sneaky fight design tricks with this.

The other option is to do a bait and switch. Perhaps the real final villain shows up, eats Zenos, and you fight them instead. Alternatively, the real final villain shows up, you befriend Zenos, and you go after the final boss together. Or perhaps you beat Zenos, then Hydaelyn calls your linkpearl to tell you to come to the Waking Sands yet again, and you fight her as the real last boss in an effort to keep your texting/teleportation charges to a minimum. But I think those are less compelling options overall. Might as well commit all the way to Zenos at this point rather than bailing out in the last moment.