The current implementation of Dolphins sidebar allows already to listen music - and i prefer that way over all the other options, to be honest, since the very most players do not fit my specification of the term K.I.S.S. and if they do, then not as Qt/KDE incarnation. Here are 2 little ideas, who set the base for the further development of this functionality: https://bugs.kde.org/show_bug.cgi?id=364956 https://bugs.kde.org/show_bug.cgi?id=364957 The third idea is now, to implement a playlist, most likely as plugin.
Dolphin has no purpose in becoming a real media player. Dolphin information panel is just for simple file preview. Plugins for the information panel is not a viable option.
Simple by default, powerful when needed. What speaks against it? If anybody implements it.
A media playlist in a filemanager is simply overkill, way beyond "powerful when needed" but in the category "every program tends to include an email client", since media playlist belongs to media players.