Francis Rufus Bellamy’s 1953 adventure novel, about a guy who shrinks to half an inch tall and adventures among the ants, strikes me as the right way to go about using formians in a game. This way, you don’t need to put aside a part of your world to be ruled by a giant ant kingdom; you can vary the encounters with battles against giant bees and grasshoppers; and you can have a boss fight against a badass shrew. Those guys are super mean and scary looking! And they can be any size from half an inch to a foot long! You can use any of the excellent official D&D shrew minis already in your possession.While your PCs are shrunk, have them make discoveries about their familiar surroundings they couldn’t normally make. Perhaps they climb into a tiny crack in the wall, and discover Diamond as Big as the Ritz, or a giant gold ring. When they return to their normal size, they can retrieve it. (Come to think of it, a ring makes some interesting defensible terrain for ant-sized PCs, forming a little fortress around one or four squares, behind which they can take cover.
The key to an interesting ant adventure is to make the ants sentient, as Bellamy did. Have the PCs choose sides in a war between an ant colony and a rival colony of slave raiders. Have them befriend ants, and ride them like mounts in daring cavalry attacks. When the PCs return to normal size, maybe they’ll have different feelings about the ants scuttling beneath their feet, over whom they now have such power.