Hey there @Talisman
Though I find it might be interesting to see some tutorials on Plugins development on S.A, I also think it’s not that easy to implement and I’m not sure that this would be a topic of interest for a large audience. Development isn’t something easy to learn and most of all it takes a very long time, even just to cover some basics. So starting something like this would involved complete series of tutorials, you can just scratch the surface in a few hours course.
As for resources, there’s a load of things available online but I think you have to decide first what is exactly you want to learn and focus on. There are different areas to dive in, and again, because it takes time, you can’t dive deep into everything.
When it comes to audio DSP development, there’s different ways to do it, different frameworks and coding languages available. Wile C++ is still one of the most efficient in terms of resources performances, it’s definitely not the easiest one to learn. Frameworks like Juce are much more easy to learn, even big guys in the audio DSP development like Steve Duda are mentioning that if they could have coded a Plugin like Serum with Juce instead of only C++, things would be much more easy for them now. While of course it’s best to be able to use both of them, Juce might be a little more easy to start with. So that’s a first bit to define IMO, which framework or language you’d like to dive into & learn, also knowing that at certain points you’ll also need to learn bits of other languages like Java, Phyton and LEA modules.
Then it’s also interesting to decide if you plan to get involved in something quite broad, with perhaps the advantage of being able to develop many different Plugins, but that will mean a much more longer learning curve, or getting more specific instead, and things like developing Max For Live modules with Max or even virtual modular modules for platform like Cherry Audio Voltage Modular could be other niche segments to take a look at.
As for some resources to start looking into this, maybe those could be helpful :
And to start learning and even deciding to get involved in a course, have a look at Kadenze Academy and here is a starting course example. There’s also many other MOOC platforms available, you can have a look at this here to start searching for some resources.
Getting into audio DSPs and Plugins development could be a lifetime involvement once you’re sure you’ve got the right mind set and skills for it, so I think it’s better to narrow it down when it comes to goals and what you’d like to develop. Don’t forget that many Plugins Manufacturers are not even handling themselves the audio DSP parts of the development, they rather hire independent developers or for larger companies they have many people, each of them dealing with a specific part for only one product and it’s hard to compete with this where you’re on your own.
And to finish, there’s not only the software side of things, I think it’s really interesting to take a look at coding for hardware modules like Arduino for example, there’s a load of possibilities to interact with audio and Midi right now and Midi 2 is now showing up, so diving into controllers might also be something very interesting and with a good potential for future projects.
Hope this helps 
Cheers !
LOL 