Right?! OMG, I forgot about that about Mjolnir! That would be perfect! I'm so annoyed Van Der Decken's amazing fruit was wasted on a POS like him, I wanted Van Auger to get it (the wrong Van got it lmao) but this would be awesome!
I still want to see Usopp get a new weapon or a DF for his kabuto or eat a DF himself too! But what should that DF be? Hmm....Or even the new weapon for that matter? Nami got an upgrade from Usopp for Zou and then got Zeus too, Usopp needs some new stuff too! Hopefully he can get some awesome new ideas for plant weapons from Elbaf, GIANT flower/plant weapons but I hope the giants can help him strong and train him in haki and such.
If we're getting some of the ultimate observation in the series with Loki or Odin, how perfect for our sniper boy who LOVES the giants and Elbaf AND who got observation haki in Dressrosa and never used it again! Hes definitely gonna need it on the BIGGEST island in the series and thus the biggest opponents too!
Usopp using Mjolnir like that would be amazing, I'd love that! Imagine it chasing Van Auger around as he tries to teleport around to get away from it? Never mind homing bullets, a homing thunder hammer of the ACTUAL Gods! LMAOOOOOO!
On a second thought, throwing Mjolnir at Van Auger might be a little over the top and kinda unfair. If we want to have an exciting Fight.
(I still want him to have it)
I think you had the right Idea, with Usopp expanding his Observstion haki on Elbaf. The geographical size is a great invitation for that. Similar to how Rayleigh was able to detect every creature living on that Island they trained on with Luffy, and he was also able to identify How many of them were stronger than Luffy, that could be a Goal worth aiming for.
Especially since, we learned from Dorry and Brogy during Egghead that there are all kinds of enormous Beasts living in the Jungles of Elbaf when they confronted the Gorosei. Which could be a cool parallel. I wonder what type of "creatures" we will get to see soon, that the Gorosei's reminded them of.
As for a potential Devil Fruit upgrade, hm. I never really thought about that. I guess I will do some brainstorming for that during the break in the upcoming days / weeks. Plant-wise, I know that the Norse people planted a wide variety of flowers for their practicality. Some plants, were used for medicinal purposes, while others, such as the daisy, were used for food.
Maybe Chopper can also get something out of this then.
In some sources, Odin's Spear Gungir was made from the wood of the world tree. It is also known to always hit its target, regardless of the wielder's skills. Perhaps you could forge some specialized Weapon for Usopp.