Julian,
what kind of tinymce plugins are you planning? What should they do?
If you are just planning to change tinymce skins or skin variants it does not require new plugins (or changes to core code).
Download or create a new tinymce (bootstrap) skin - see for example https://github.com/gtraxx/tinymce-skin-bootstrap or attached file and unzip/move it to lib/editor/tinymce/tiny_mce/version_of_tinymce/themes/advanced/skins/
You can activate this skin from site administration in moodle 2.5:
Add to Site administration > Plugins > Text editors > TinyMCE HTML editor > General settings > Custom configuration
{"skin" : "bootstrap"}
and press Save changes
(supposing you don't want to edit any core files)
