Let’s talk about licenses and what you can do with these

Hi, I’m a WordPress developer since a lot of time and I like to customize plugins.

While I develop many sites in the week, I always go through the same situations, like using many plugins that could be better coded, or, plugins that should add multiple useful features and their developers don’t care.

In these cases I like to edit those and make my own version, however sometimes building a “new version” of a bad coded plugin is a really bad idea so I end developing a totally new plugin from scratch that does the same and more.

Also, sometimes I have some good ideas for new plugins and when I go to the market, they already exist but were abandoned or are very expensive.

My questions are, having in mind that WordPress (and its repository) is under GPL2:

– Are ALL plugins/themes outside the wordpress repository GPL2 too? or they can use another license?
– Can I take a plugin from the repository, make a new version (with more or less changes) and publish my own version in the repository? If this can be done, can I publish a paid version of this on my personal store too?
– Can I take a paid theme/plugin, edit it, and distribuite it for free (or as alternative, sell it)?

I hope somebody can answer my questions, thanks.