Can I make my Mac re-adjust to the original brightness when I plug my power cord back in?
I use a program named EventScripts which among the events it monitors, there are some Power Events, one of which is the Power switched to mains event. It uses AppleScript scripts to preform the action(s) you want to have happen when a given event is triggered.



It is a paid app, $3.99 USD, but does have a demo version you can download to test with before purchasing.


Mac App Store Preview (US)
Download for Demo




Note: Other then being a paid user of this app, I have no affiliation with its developer.