The [Otontin Slinger] could possibly have a “Damage Over Time” effect (DOT) where they make enemy units bleed overtime. I remember reading on the Wikipedia that the Aztecs did apparently use hand molded clay balls for their “Tēmātlatl” (Slings) which were filled with either obsidian flakes or pebbles.
There could also be a card for them called “Furious Slinging” which will decrease the set-up time for the [Otontin Slinger] before they attack, but there will be some kind of trade-off. The name for this card could be based on the notes from the conquistador Bernal Diaz del Castillo who did apparently describe that the hail of stones flung by Aztec slingers was so furious that even well armoured Spanish soldiers were wounded.
I suggested this in a post that I created near the end of last year.