Key molds have no intended use other than to craft their respective keys.
Key molds have just a single, very straightforward purpose. Saving up spares won't do anything except allow you to make more keys to open similar chests in new worlds (or to make people who are grinding for them jealous).
As for possible future uses: update 220.127.116.11 (currently only on the Desktop version of the game) removes key molds from the game and simply has the biome keys themselves drop from enemies. Any existing key molds are automatically replaced with their respective keys (Source). If this change is brought to other versions of the game, obviously the Key Molds will have no alternate uses.
Even if the other versions of the game don't get this particular change...developer intent as to whether they will ever have another use is off-topic for Stack Exchange Gaming, but simply looking at it logically and considering what they are and what they're used for, it seems unlikely they will ever be given additional uses.