A very useful rule would be one that prevents power-saving features (screen dimming / screen switch off / sleep) while the window/application it's set for is:
-RUNNING (as a process, could be in the tray, or minimized),
or OPEN (the program should not be minimized, should be visible on the screen, even if not focused or under other windows),
or FOCUSED (prevent power-saving only while this window is in focus)

Ideally, it should be customizable to let you select the level of power saving allowed in a dropdown menu. Here is how the option might look like: [Things in brackets are dropdowns]

-Prevent [Dimming / Screen off / Sleep] while app is [Focused / Open / Running]

This would be great for many different use cases, for example to prevent sleep while a music player is playing, but to allow the screen to turn off; or to prevent screen off while in a RDP session, but to allow dimming. Or to prevent dimming while playing a game with a controller (they don't count as input and can't wake the PC).