Work with new tech.
Stagnation is a challenge. Regardless of how fun it can be to do slight variations and improvements on the same thing it is very easy to get stuck in one area of thought.
I believe most people who read this know what kind of development I do for a living, and I obviously put a lot of effort into improving my skill with the technologies related to that.
However, there would be a big problem if that was everything I was doing.
Continuously working on different things is what keeps the imagination fresh as it makes you view things differently.
There is nothing like doing a project in a completely new language or with a completely different major framework to make you think in new patterns.
Writing code in Haskell will inform how I write C++ and to think about what pure functions without side effects can bring. Working on things in Node can make me think more about how to write async stuff. Working in Io makes me think more about dynamicness and what function calls vs message passing actually means.
These are all just examples and there are more variants of this than I could ever think of. In fact, some of the situations where working on a certain technology helps with becoming a better developer overall will not necessarily be apparent but rather be a “back-of-mind” thing.
I also really don’t believe in that you have a finite number of programming languages that you can work well in. Learning a new programming language is not going to kill that 6502 assembly knowledge that you have been holding on to for 20 years more than not using it is already doing.
So, try new stuff… If you can’t for work then do so for yourself, just remember to make up reasonably sized projects because “just testing” languages tends to be very limited if you don’t have an actual project.
This is what I’ve been doing for a long time to keep evolving. I try to learn at least a few new languages every year and while not all stick they have all been valuable. It also really helps when I for whatever reason have to bring out something obscure and work some magic.