Dipping an item into any fountain may erodeproof it with low probability. (Or else have this only apply to magic fountains with somewhat higher probability.)

Add as rare possible fountain effects erodeproofing and blessing a dipped object. (Possibly only in magic fountains.)

Allow magic fountains to be specified in special level files.

One of the Oracle’s fountains is guaranteed to be magical, but it’s randomly placed among the four fountains.

The scroll of detect magic shows magic fountains on the level as well as magical items. Fountains discovered this way are then displayed as “magic fountain” when near or farlooked. Wizards can see them as magic fountains already without needing to detect magic.