I think it would be nice to give moon dial practical use, as well as opportunity to player to obtain water regardless of worldgen luck relatively close to desired farm location. In order to not make obtaining water too easy, moon dial could act as water source only during full moon phase of the cycle, which means 1/20 of the time. I don't think that would break balance of the game since water is important only if player tries to grow giant vegetables or obtain as quick as possible some specific seeds from random ones; moreover, currently sources of water don't really impact base location choice since multiple watering cans can be crafted and used to store water, not to mention other water sources like melting ice and rain. That kind of change also would encorage to pay attention to the world more, and would be quality of life for the ones willing to farm in winter (currently there is nothing difficult in going into caves even in winter for a few minutes, or wasting a stack of ice to refill watering can).
Another idea for moon dials is to make them accept bath bomb and generate glass during full moon if bomb was used. Player still would need to visit lunar island to relocate lunar trees and to use corresponding crafting station.
In current beta after defeating moon champion selestial orb acts as crafting station for all moon-related items, but lunar island is still necessary to visit to assemble three altars, summon and defeat this boss; moon shards are obtainable via lunar grotto without need to sail to the lunar island, so it's not like opportunity to mine moon dials for glass would remove something unique from the game, in my opinion. Completely new interation with moon dial also could be interesting, since right now they serve only aesthetic purpose + remind about full moon at the beginning of the corresponding day, which is easy to predict anyway (unless resetting moon cycle after defeating moon champion is intended result).