I would pay actual money for the following to be in a mod (sadly it's breaking Minecraft's EULA if you do sell your mod but still):
It distributes mod ID's evenly so there are no conflicts
When Minecraft crashes, it will display the crash report on screen so you can go back to Minecraft without loading it up again
Get rid of annoying chat messages from mods (configurable)
Reduce search bar lag (the compass tab in the creative inventory)
If there is duplicated items it'll remove them (say Tinker's Construct adds copper and so does Immersive Engineering; it would remove the more rare version of copper so that way it will stick with just one type of copper).
A clickable widget when you go to the menu (ESC key) that reloads all chunks around you.
Make the mod configs in game so you can change them whilst playing (and be able to save them too).