If there was one tool you would put in Gimp what would it be, why, and how would it be used???

A plug-in/Filter installer - right click shell type.You point to the downloaded plug-in and a dialog pops up asking where you want it.
You get a choice of creating new menus, or using existing ones.I get so tired of editing scripts so i can place a layer filter in the layers menu.
I do not know if it's possible as the script register area would have to be overwritten probably.And EXE's would go where ever the developer decides.As they are a binary.
Maybe it's time for a new type of menu system ( folders within Gimps Plug-in and Script folders.)
Gimp could then see those folders/directory tree's on load and sort your filters appropriately.

I dunno maybe possible that way.

A built in ability to use PS plugins, without needing PSPI or any other "extra"programs.

Also a good visual distorion tool would be sweet!

What PM and Rod suggested. GIMP plug-in accessibility is the most crucial. Then improved plug-in window views.