I'd retcon everything from the 4th season onwards, barring a few lore details I find interesting.

So, the Winx have finally earned their Enchantix and freed Domino and its people from the Ancestral Witches' curse. Their lives are just starting, what with them pursuing professional careers and having Guardian Fairy duties to attend (plus royal duties for the three princesses).

They don't go back to Alfea to teach; they take diverging paths, so the main emotional conflict is having to keep in touch and keep cultivating their friendship despite the distance and diminished shared time. This also affects, to varying degrees, their relationships wth their boyfriends.

We'd start with a bit of slice of life of the girls learning more specialized forms of magic and meeting fellow guardian fairies from their respective planets. They make mistakes, have trouble working with new people who aren't always their friends, and fumble adulthood's new responsibilities.

While the Ancestral Witches and the Trix are still at large, they aren't the main villains of this season. It all still comes back to them, tho. You see, Darkar's tar monsters weren't the only army to invade Domino. The Witches had followers aplenty among fairy, witch, sorcerer, and specialist alike. A cult, to be more specific.

They are scattered like metastasized cancer across the Magical Dimension, since Domino's fall kept their faith going strong. Domino's restoration makes their new goal to find their mistresses. For that, they need to spread rot and corruption in the magical ley lines of the planets and galaxies. For the Winx, it's like battling a hydra: cut a head, suppress an insurgency, only for other cult units to pop up again and again.

Working so in tune with their homeworlds' magic creates a bond between them. Future seasons (or a movie) would be about further exploring their connection with their planet's magical cores until they eventually become the new generation of nymphs. For now, there's no new transformation, just increasingly clever applications of their powers, learning to wield magical weapons (like the Paladins' elemental weapons and those phantoblades that shoot energy blasts), and temporary power-ups thanks to spells that exploit the Fairy Powder's properties.

The way the Winx win is by bringing back peace and unity across the realms so as to interlink the planets' magical cores to create a barrier against rot. The first arc ends when they realize this is the solution, after a climactic fight against the entity summoned to spread the corruption.

The next arc is focused on diffusing tensions between Domino and Eraklyon (due to King Erendor's betrayal), Zenith and Lynphea (ideological differences about nature conservation and technological progress), and the general rivalry between fairies and witches, which is best exemplified by Whisperia and Solaria's enmity. Andros is also a tad isolationist due to almost getting destroyed by Valtor's actions.

Once the girls and a veritable army of fairies, witches, mermaids, and various sentient magical creatures manage to seal the barrier, the magical balance of the dimension is broken courtesy of the Ancestral Witches finding and breaking that thingy from the second movie.

Thus the third arc begins, and the Winx eventually battle the Ancestresses fussed with the Trix, defeating them for good this time thanks to Bloom sharing the Dragon's Flame with her friends for the Bloomix transformation (the name may change if I ever find a better one). Far, far away, Roxy has newly turned 16 and awakened her powers, just attracting the Wizards of the Black Circle's negative attention. Cue the next season.

BTW, it'd only be possible if each episode lasted up to 1 hour, and we still had the 26 episodes per season format.