MIDI Lab Version 1.2 is a big step forward. It is MIDI-centered, but can now host VST instruments and effects (VST2 and VST3, 64-bit).
There are many internal MIDI plugins (named blocks) that can manipulate and generate MIDI data flow.
You can receive MIDI data from an external MIDI port (or the internal MIDI player). Then you can connect the internal MIDI blocks and external MIDI VST plugins (eg. arpegiators, chord generators, ..) to each other as desired. At the end the MIDI data flow can be sent to an external synthesizer or internal VST instrument.
You can also connect audio outputs from VST instruments to VST effects, mix them and send them to the audio out port.
The GUI of the VST plugins are each presented in a tab but you can create an own tab where you can add all GUIs of the plugins you want.