A perennial question. There's no "right" answer - it depends on the sort of flying you want to do, and how much time and money you can devote to it.
There are two pieces that can relatively easily bolt on to your PPL: the IMC Rating, and the Night Rating (or whatever they are called this week).
They can be done in either order. My wise instructor, who taught me through the PPL, advised the Night Rating first, so that's what I did. I went straight on to the IMC after that. That worked fine for me, but I wanted to be able to go touring with an aeroplane and have some chance of getting there and back again.
Then there's aerobatics, and variable pitch props, and retractable, and tailwheel, and ...