Nice project!
But expect legal problems. There are ARM versions that you can implement freely, but they don't (yet?) include anything with Thumb.
I think Thumb is old enough that any patents covering it have expired.
Maybe, just.
Patents are supposedly 20 years from filing, but you'll notice that for example MP3, introduced in 1993 (a year before Thumb), has just a few weeks ago lost patent protection. Fraunhofer announced that "MP3 is dead". What they of course meant is that MP3 is now freed, and it is Fraunhofer's licensing business model is dead.
ARM had to license some SuperH-related patents from HItachi for Thumb, Those will be expired now.
MIPS16 from 1996 uses basically the same ideas as Thumb -- 16 bit instructions pretend to have odd addresses, switch on function call/return (or at least change of control flow), only 8 registers available out of 32. You'd imagine they probably had to pay money to ARM, but I don't know about it.
RISC-V has an optional 16 bit instruction encoding, but they do it quite differently, using a scheme similar to that first used by the IBM 360 fifty years ago with a fixed two bits in the instruction indicating whether the instruction is 16, 32, or 48 bits long. So you can freely mix instructions of different lengths. Thumb2 of course does something similar.