I created a huge thread of suggestions for Pelles C a couple of years ago. After using Pelles C for several years, my list is smaller and I'll be creating three separate threads here.
What I want the most is an updated autocomplete. This feature in Pelles C is near the level of what is included with Microsoft Visual C++ 6.0 and Visual C++ 6.0 is ancient.
Considering the fact that Pelles C is only plain C, the only real addition to the autocomplete that I'd want is suggestions for what function you might be typing for that you can accept as correct if it's what you want. It would be especially useful for the Win32 Windows API functions and the functions that you create in your code. The Pelles C IDE already offers suggestions for structures.