SchismTracker is a reimagining of a classic MS-DOS tracker (Impulse Tracker, which in turn was an alternative and expanded take on the design decisions made in another classic tracker, Scream Tracker 3), made cross-platform and compatible with modern systems. I included the smiley there because it is so retro , but I'm sure it can be inspiring to some! (There's also a Fast Tracker II for modern systems, https://16-bits.org/ft2.php , hehe).
PureData is a free environment very much related to Max(/MSP), by the original Max developer. Google both and check whether it's your cuppa tea.
CSound also harkens back to the time when these modern DAWs of ours were still sci-fi; it's a classic computer music / sound generation and manipulation toolset with its own programming language and deep libraries of things to try if the approach itself inspires you (the same goes for PureData as well). There are also front-ends like CsoundQt that make experimenting with it a lot more immediate: https://csoundqt.github.io
All three are... I guess pretty dang oldskool approaches these days, in a historical perspective, on that ad hoc list of mine. Acquired tastes and so on , but interesting!