Thanks, that means a lot to me. I know most people don't feel they should pay for software. It's too bad but I understand it. It makes being a small software developer very nearly impossible if you want to earn money doing it.
As for the plugin we'll see if anyone else has any ideas. Probably you won't be asked for anything "simple" though.
You could make a generic serial port plugin. I have not done this since there is usually too much other customization required when using serial so it just makes more sense to use the python plugin but some might still find it useful, and you'd probably learn a lot doing it without it being too difficult or complex.
A plugin for a serial or network audio receiver might be good if you have one other than the Onkyo.
I know there will always be users looking for plugins for any home automation devices or software out there that we don't already have a plugin for (Vera, Hue, X10). But that probably won't be simple.
People are often interested in easier ways to send notifications to their phones etc.
Some people are looking for a plugin for Plex but that will be a lot of work.
Setting up daily timers maybe... That was on my to do list, although I find the GgCal plugin better for that anyway. Some people might want something that works without internet access.
Obviously you should do whatever interests you that isn't too difficult.