DirectMusic was a fascinating platform. It's interesting that while we've gained fidelity (in game music) by using mp3/vorbis streams, we lose some of interactivity and power afforded by sample-based formats. There are some interactive effects, dynamism, and music tricks that games based on MIDI and MOD formats/platforms (such as DirectMusic) were able to pull off that I'm not sure we're likely to see again in mainstream/AAA games.
It would be interesting to see if an open source toolkit like this might provide fodder for an interested indie or two to explore cool/weird soundtrack ideas.
I agree, it was (and still is) incredibly fascinating, especially now that I get to look at it from very close. In writing this compatibility layer I am indeed being inspired a library with a similar intent, maybe with a more modern approach. We'll see!
Since you are looking for ideas to explore in a new library: You may already be aware of it, but it might be worth looking at SCUMMVM's reimplementation of "iMUSE" as well, which was LucasArt's similar engine used in their games, for interactive MIDI control based on game actions. (There were some really clever LucasArts' three composers did in the first iMUSE game, Monkey Island 2, especially, that decades later still hold up as gems.)
It would be interesting to see if an open source toolkit like this might provide fodder for an interested indie or two to explore cool/weird soundtrack ideas.