MIDI processing VST plugin

I want to build a plugin that makes chords out of a (monophonic) live MIDI input. So just MIDI I/O. Do I just modify the existing MidiBuffer? Because in a simple test, I’ve just typed

msg = mbIterator.getNextMessage();
if (msg.isNoteOn())
  lastNoteIn = msg.getNoteNumber();
  msg.setNoteNumber (msg.getNoteNumber() + 1);

But when I chain 2 of those plugins after eachother, the second will display the same key as the first, still.

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Thanks, that explains everything I need to know!