Flish

I'm for this specifically because it would help make lime more accessible to the average player without extensive travel without making limestone/chalk completely worthless to search for- You can get lucky and find a few speleothems for some limestone in other areas, but realistically for long periods of time (or making any amount of plaster) you still want to go out and find a limestone/chalk area.

I would honestly just like to see bogs (and in contrast, fens) become a thing in general, to be honest. The way peat deposits form currently is super weird, and it'd certainly add some more visual distinction if there was areas of 'low PH' that form bogs with peat, goethite, and unique bog plants like cranberries, horsetails, and new plants like venus fly traps or pitcher plants.