Patternmaking, both basic and creative: The Shapes of Fabric
Once upon a time I used to follow the blog of someone who worked in the fashion industry who would write irate posts on fitting and quality issues, teaching me in the process why ready to wear trousers never fit properly, exactly how to set a sleeve, and why T-shirts sometimes start to turn themselves into corkscrews once you've washed them. I'm pretty sure this is not that blog, but I think I found it when I was trying to find the aforementioned one again.
The Shapes of Fabric is a really cool blog with a mix of basic guides like drafting pattern blocks (a base pattern you can use to create more complex garments) and really fun patternmaking experiments that are basically practical geometry.
If you're going to use older sewing sources, it's often really helpful to have at least a basic grasp of what pattern drafting is and how it works, so I'll be linking to some tutorials on how to draft some basic pattern blocks (bearing in mind that I've probably not used this source in particular, but the idea is more or less the same however you explain it):
how to use basic pattern blocks
The site also has a bunch of helpful guides, like these ones:
sewing plackets and collars
There's also some really cool posts on dart manipulation, flounces, ruffles, "patterns that create patterns" (if you want to go off in a retro futurist way this kind of thing would be neat to incorporate), funky things you can do with sleeves, tucks (v useful if you're into Victorian stuff), detachable collars. Basically, go have a look around and get inspired!