Hi there!
I can take a look at this visibility toggling feature! Should be easy to implement!
When you mean, variable-based timers, do you mean using the default variable as a timer? Like, raise +1 variable value every 60 frames? This for sure will work with parallel common events, and across different maps too.
I should warn you that if I release an update, will be only for the MZ version, not MV.
Also, some piece of advice: If you are really new to the software, forget about plugins for now. Learn the engine first, understand how everything works, and every event command. When you are feeling confident that you learned it well, then you move on to use plugins.