For those crazy enough to try:
Don't build a workstation, build an ecosystem.
Scope aggressively and specialize. Learn to say no. Less is more.
GUIs are a black hole of productivity, don't sucked into them.
Make sure offline rendering happens on day one.
Do NOT multi-thread the audio rendering component.
Choose standards you intend to support very, very carefully.
Question every DAW design pattern or paradigm. Don't ever do anything by default.
Make sure everything can work headless or without a GUI.
Emphasize extendability. When in doubt, just build a canvas.
Manage your dependencies. Don't go crazy with packages.