PB is complete. - Well...Sortof. You can make a game with all of the necessary tools. You can easily make games with pre-built stuff only with simple custom stuff. But when it comes to complex stuff, PB has missing pieces. Those missing pieces are what this topic is about. Here are the ones I can think of. If you find any I missed, let me know, and I'll edit this topic with those things added!
Visual HUD editorFully customizable Menu Creator
GML based commands (I'd like to expound on that a little. Find the * to find out what I want to get into detail on about it)
Custom Character sprites
HD graphic support
Camera zoom
Physics in numbers for all objects
Custom Object creator (all custom item, block, enemy etc combined with GML inspired commands plus more to create fully customizable stuff like in Game Maker Studio 2)
Fully customizable physics and from scratch (done with GML based commands)
I know I've missed a lot. But that's all I could think of. I hope TingThing will refer to this list as it grows. Whenever he adds something, I'll either take it off, or just mark it DONE.