In case Amarok is deprecated and Clementine is not far from deprecated, will Elisa have an equalizer in the future? or better still kde Plasma could have a universal equalizer in the volume menu, a little like "pulseaudio equalizer" but native kde and divided by single applications that can be opened in the audio menu (which is perhaps better since we use a lot of youtube etc ...)?

Doubt that Elisa will get proper equalizer support. Posted it here somewhere https://forum.kde.org/viewtopic.php?f=116&t=174836, but if you do a lot of music libraries and such, Strawberry ( clementine based) is a good option. It has more functionality and is better updated. Personally I use Audacious as player. Hell, even lameass vlc does a better job than Elisa. As far as systemwide equalizing, there are a few options availbale, but in my experience, results weren't so satisfying. Easy effects is such a one. About YouTube in specific, I believe there are equalizer extensions/addons in the stores.
Never mind the link for an alternative suggestion. They get deleted anyway.