As for the fetchlands, I actually think they should not count for mana statistics, at least not automatically.
I mean, I think the <source> tag is enough to allow the users to customize their mana-base evaluations.
Let me explain:
With the new algorithm,
Verdant Catacombs counts as Golgari while
Prismatic Vista counts as an any-color land.
At first look this could be acceptable, but in fact it's false.
The problem is the fetchlands can grab duals while
Prismatic Vista only gets basic lands.
For instance, everybody knows that in a tri-color deck, any in-color fetchland can get any dual, so they effectively count as any-color.
But if we're playing 5 colors things get more complicated.
Take this deck as an example:
https://deckstats.net/decks/54477/1203990-domain-cascadeHere
Verdant Catacombs can grab any dual, so it could count as an any-color.
The same is true for the other fetchlands aside from
Scalding Tarn. In fact,
Scalding Tarn can't get white mana.
No need to say here
Prismatic Vista would be terrible since it could only grab a
Forest or a
Swamp.
Fetchlands are very dependant on the decklist structure, that's why I'd let them out of automatic calculations.