Wuppo Definitve Edition Update (17 September 2019) Co-Op:

We added co-op (couch), where the 2nd player can play as Denksnavel. Our main goal of this feature was to provide a little help to players that need it. So if you’re having trouble with a specific boss, or just need an extra pair of eyes to look for clues, make your friend / parent jump in at any time with Denksnavel.
Visual updates:

More dynamic lighting. Lights can cast subtle shadows on the terrain. The characters also receive light on their bodies.

Fog: Sometimes the area gets wet / foggy.

updated maps: A few maps have been refined.

The camera now zooms in at some of the more important cutscenes

Some colour palettes have been refined and the game is a bit brighter during night time.
Audio updates:

A couple of brand new songs have been added to the game, and many songs have been retouched slightly. We tried to create a better structure for the soundtrack from beginning to end, with some more recurring themes, highlights and consistency in quality.

The important NPCs also have more varying voices, giving them much more personality.
New / changed areas:

New trainstation: Wisest Wum: Ever wanted to test your Wuppo knowledge? Well, now you can. Wisest Wum is a quiz show that gets hosted at one of the train stations. You can earn (or lose) a lot of smurt by contending.

Redav Kned’s Guesthouse has been made easier. The original difficulty is still available when playing the game on INSANE. The ritual has also been reworked to give much better feedback to the player and help solve it.
Boss changes:

The final boss has been completely reworked. It’s super important to us that the game ends in a spectacular manner. After watching players fighting the boss and receiving feedback, we ultimately thought we could’ve done a much better job, so we just started over. We tried to bring back everything the player has learned from previous fights. We really hope you enjoy the renewed fight.

King Blione: has an extra attack

Blafknoft’s attacks changed a bit.

All other bosses received some (smaller) changes as well. Either balancing, or improving player feedback.
Happiness hints:

Our mutual friend Toekomsthakker now has a new feature, where he gives you hints for any of the happiness that the player is missing. A great excuse to spend your Wondersplenk Coins.
New items:

We added a light bulb and a blusser outfit. The light bulb makes you glow in the dark for 7 minutes, and the blusser outfit can be used as a skin when using bucket.
Added & adjusted dialogues:

We saw an opportunity to improve the quality on some of the dialogues. Most of the changes are modifications on existing dialogues, but there’s some additional dialogues as well for the new cutscenes / new area.
Gamepad/Controller controls:

A new shortcut menu has been added when playing with a Controller. It acts like an inventory with 9 slots, but it’s very quick & easy to select the desired item. Also, without the need to memorize which specific buttons you have directed to specific items. The game also runs in slow motion when using this feature during boss fights.
Additional QOL changes

Game now pauses if you browse your inventory during a boss fight.

Added a couple more Autosaves throughout the game.

Losing an item doesn’t make you lose the hotkey until you change it. Which is useful if you just throw your gumgumgun in the gumcore machine and regain it within a couple of seconds.

Game automatically adds hotkeys for new items (optional)

Reworked the options menu to be easier to read, as well as providing some additional graphics settings.

Blieken instrument controls have been made more flexible. Some people didn’t realise they had to hold the action button while picking a note. Now you can use it either way.

Boss run score: one of the scores now indicates how many times you’ve been hit rather than how much damage you’ve received.
DLC = free

As it is now called the "Definitive Edition", all DLC is included for free. The Original Soundtrack, the Bonus shops and the Wuppo Fun Pack are now included into the base game. The price of the base game stays the same.
Performance:

The game's performance has been massively increased and also uses "delta time". Which comes down to that it handles frame-drops better. Keep in mind that you can tweak the settings in graphics to ensure 60fps. The game will give a warning if you dip below 60fps.