After defeating a champion, the character who manages to get the final hit off would be well deserving of a slightly overtuned weapon, piece of armor or ability augmenter just to add some distinction between character prizes. May add to the replayability of the champion as you try to support each character to get the final hit off or just try and solo the boss with the cast.
Wigfrid bias out of the way first, Mjolnir. This can be crafted with a combination of thulecite and moon ores or glass and have it do constant lightning damage, big fat damage stat and a very durable hammer and efficient hammer if you so choose. The catch is only wigfrid is worthy to lift it. Just for lolz mighty wolfgang would be able to lift it when mighty, but moves at statue speed.
Willow could have a staff that can light every campfire in her radius instantly and have them burn for half a day to a full day. I just think that would be pretty badass and practical enough. You could argue that it would be too overtuned, having that much light, heat or cold when using endo pits, but you've beaten a lunar champion and gone through all the pain in that world. Light, heat or cold wouldn't be such a struggle.
My suggestion for woodie would have been summoning a full moon any night, but the current beta gives him a generous amount of full moon. That suggestion is shot to :manure: so perhaps just a basic 2nd tier of his transformations or a lucy that can be thrown ala forge. Add some glass and moonrock and you could have a lucy that does 27.2 damage that functions as a boomerang if you so chose. In my opinion, a pretty modest prize for an endgame boss kill but still probably fun to play with.
The rest of the characters I don't really have the most solid of suggestions for. Whatever i can throw out won't be that sincere, but name some cool things you want your character to have if you so desire, but here's some insincere and top of my head spitballs;
Wickerbottom; a comfy bed craft where she can finally sleep lololol
Wurt: Trident upgrade so she can use geysers on land
Wes: Balloon mech with 13 damage aoe punch.
Wormwood: Can plant any flora anywhere including cacti and spiky desert/swamp trees. reeds. (Probably too powerful, i'll admit, but killing final boss...)
Winona: Her crafts but with wheels only she can move.
Warly: Oven that can cook stacks of food
and finally, wilson. A doctorate so he can finally say that he is a true scientist lol also lmao.
the first 3 suggestions were for characters i have a stronger bias for than the others so they're a little more fleshed out but the rest are divisive to myself even if I do like some of them.
If you have any changes to the suggestions, open forum, let us know.
Wolfgang: boxing gloves so he can punch mucho rapido.
WX: MOONLASERCANNON
Max: shadow blowdartsmen. 200 damage for just targeting an enemy. not srz.
Wendy: Flower upgrade to swap consciousness with Abigail (I'm pretty on board with this one, It seems pretty cool to me, but not a wendy main. Can't say the same for the rest)
Wortox: A staff or wand to teleport creatures to point a to b limited to the distance of his line of sight (I like this one too)