Arno Puder and Andrew Quinn have developed a Model III on a board with TRS-IO built in. For information, gerbers, etc., visit https://github.com/apuder/PocketTRS.
Arno has also made his TRS-IO working with a Model I. For information, gerbers, etc., visit https://github.com/apuder/TRS-IO.
Richard Lamb has been creating some new TRS-80 software. You can find his efforts at https://github.com/voidware.