Super Mario Odyssey is a pretty great game. So great that you'd be hard-pushed to find anyone who has played it who has anything negative to say about it. That might well change after you read this and you're reminded of the handy water-filled backpack from Super Mario Sunshine which has now been modded into Odyssey.
Known as the Flash Liquidizer Ultra Dousing Device, (or FLUDD so I don't have to type that ten more times) the backpack can now be used in Odyssey. Created by JustKant (thanks, Nintendo Life), Mario can don the FLUDD and do almost everything he could do with it in Sunshine. Its Hover, Rocket, and Turbo nozzles are all a part of the mod, with the only feature seemingly not present being the ability to squirt.
In defense of JustKant, that particular use of the FLUDD might have been pretty tricky to pull from Sunshine and add to Odyssey, regardless of how helpful it would be in the Switch game. That doesn't mean the FLUDD isn't useful without it. As the trailer for the mod shows, its other nozzles can be used to hover high above Odyssey's levels, and it even appears to work when Mario reverts to 2D, even though in that scenario the FLUDD can't actually be seen on the screen.
If you want to try Odyssey's FLUDD mod out for yourselves, combining the plumber's GameCube era with his current Switch one, you can find everything you need to know regarding how to do that in the description of the video above. Do note that the mod will only work with the v1.0.0 version of Odyssey.
Whether you're using the FLUDD in Odyssey or going back and replaying Sunshine, while Mario has discovered a number of new abilities since, it seems odd that the backpack has never officially returned through another installment. The same applies to Mario Kart: Double Dash, another unique spin on an iconic Mario series left unvisited since the GameCube era.