We abused hblank to death in Infinity for Game Boy Color (unpublished in 2001, later dumped on GitHub), for things like parallax scrolling, tilted overworld map, screen transitions, and scrolling inner text.
If so, what approach are you taking for the toolchain? Keeping the existing version that had been used/modified? Updating to newer tools (and code changes that may go along with that)?
Can definitely see wanting to keep the compiler and code in a known state. Though modern SDCC generates code that's probably 50% faster in some cases and with 90% less bugs than versions from the early 2000s.
Lots of great Game Boy releases have been coming out.
Edit: add a little more.