In a 300g, you could have a panther grouper. Not sure how it would do with the tang though.
If you would want to go more for more of an aggressive or predator tank,
I would bring the powder blue tang to the LFS.
Then, fish choices include
-Panther Grouper
-Marbled Catshark
-Volitans Lionfish
That's just a list to get you started. There are many more choices
If you wanted a more peaceful community tank, you would definitely have to leave out the catshark and instead look at
-Tangs
-Angelfish