You could try stocking some GIGANTIC bluegill in hopes that they spawn and kick start the food chain. Its hard when you have bass that are that large already.
Tilapia could be another good choice. If you get some really big Tilapia they might take the heat off of the bluegill population and help them take a hold in your pond.
Golden shiners could be another good choice, but again you will need massive adults.
One thing that you will want to consider before adding any forage to the pond is how many bass do you have in there? You may want to remove SOME of them to give your forage a better chance at survival. In fact, if you pull out any bass under 12" you will be pulling the bass that eat the smaller bluegill. Hopefully with a reduced number of smaller bass your bluegill will make it to a size where they can spawn.
You will also want to consider evaluating the cover for forage in your pond. Bluegills can use large rock piles, pallet stacks, christmas trees, PVC structures, or pretty much anything you can find. Add some cover and that will give them an edge too.
