Does musescore fit your needs? If you want a piano… you should probably get a midi input keyboard. If you don’t need 88 keys and real-time playing, you can deal with less octaves.

Do you mean a VST? There’s a good list of open-source VSTs here: https://github.com/webprofusion/OpenAudio

Well… if you want the very minimum necessary to play piano, I’ve written this tool to do it with just a MIDI keyboard and a modern browser.

http://tools.fabioiotti.com/midi-synth/index.html https://github.com/bruce965/midi-synth

I guess the closest to a decent FOSS piano plugin is MDA Piano, or perhaps search for piano samples. Perhaps someone has created a decent piano preset for the dexed FM synth (but will probably sound very 80s). I’m using pianoteq (unfortunately proprietary, but it has native linux support and sounds good).

You could try dexed, it’s a YamahaDX7 clone https://github.com/asb2m10/dexed/releases