Well a sub would be your next step up, but it's best to run a sub off an amp.
You don't want to run a sub off your head unit.
So why not by a cheap 12" JL W0 and a Alpine 5 channel amp.
You could run your highs and sub off the 5 channel.
At this point the only up grade would be an EQ and more/ better of what you already have.
More/better, like an eq and amp for each speaker and the skills to tune it

. Yes, it's over kill and most don't go there. So just do a 12" and 5 channel and be done.