We have to make it easy for plugins to have their own Configuration file, the same way they have their own translation file, etc.
now available via the Option table; better solution than having an option file by plugin. see function Piwik_GetOption($name) and function Piwik_UpdateOption($name, $value, $autoload = 0)
I believe we just need to write a small FAQ question about how to store and get values like in a config file, using the option mechanism.
Can we add a cache layer to this? I envision future tracker plugins may want to access their configuration.
Actually it is best not to make it easy for plugin to have config file, or they might be misused. Settings should be stored in the DB using Piwik_SetOption helper, and settings should be editable in the UI. For other constants and magic numbers, fine to have them as class internals.