It would be a drastic change to go from a dual-type to a single type and that hasn't been done before, which I think would be so cool to have happen. IRL Evolution doesn't always just add "positive" traits and remove "negative", it can take away "positive" too, like STAB, if something else is favored. Aggron loses its Rock-type upon Mega Evolving (which in addition to Digivolving, I just realized Mega Evolving is also very similar to the ultimate aliens in Ben 10). The introduction of regional forms was a huge step for Gamefreak in mixing in more scientific concepts in how evolution actually works. The riddle does say that the Pokemon needs to "survive" a certain amount of battles.
I would love it if it's regular, Normal/Flying Farfetch'd that evolves, but I'm here for a Galarian one too. However, I wrote a few posts about this in the General Discussion thread. We have 3 Pokemon that completely change their types upon evolving: Eevee, Cubone, and Azurill (Pre-Fairy type). Why not have more?
Maybe similar to Marowak becoming Fire/Ghost (I wish they'd just make OG Marowak Ground/Ghost) to protect its young from an abundance of Grass-types, Farfetch'd pushed itself to fight harder in battles due to an abundance of Rock-types and became pure Fighting. It would lose the Flying to avoid the Rock weakness. Rock resists Normal so if Farfetch'd was faced with too many Rock-types, having Normal STAB doesn't benefit it and it would have no reason to carry it into a new evolved form.
We've also had location-based evolution before. I could see them having some Pokemon evolve into a new pkmn with the only condition being that they are in the Galar region. Similar to Pikachu, Exeggucute, and Cubone (though it also requires night) in Alola. It is also currently impossible to evolve Eevee into Glaceon and Leafeon, Magneton into Magnezone, and Nosepass into Probopass (Vikavolt would fall into this as well) in Johto or Kanto, effectively making them regional evolutions, but only to certain regions. Having region-locked evolutions is an easy way for Gamefreak to avoid having to come up with new methods of evolving or complete new forms.