You are looking for a standalone vst/vsti host app. I don't know if there is one for iOS. But there are some (not a lot) for Mac and Windows. The problem is that vst/vsti is a plugin technology oriented to big DAW applications, which are vst(i) hosts in addition to record/edit/production tools.
But if you open your mind a little more, now that you realized that you don't need a DAW to enjoy your music freely, then maybe you discover that you are looking for good realtime MIDI Soft Synthesizers, without limiting yourself to VST(i). Maybe you are even disposed to explore Linux too.