What to do with my car audio?

-I currently have an 09' Mustang. It has a stock stereo system, it has really good bass already, but I want to upgrade it more. But I dont want some 50 pound speakers weighing down my car or taking up trunk space. Is there anything else I can do? Im open to anything.change the head unit for an aftermarket...not a cheap walmart one. And replace all the speakers. Almost all brands that make bigger subs will also sell smaller speakers.