When I was 15, I created my own programming language following old tutorials written for Turbo Pascal which I translated into C. I followed principles from that, PHP's internals, and later the Wren language which I hope is still around. This is something that it'd be fun to work with in the future, specifically languages that are compiled to bytecode and run on some kind of VM. Oh! I also wrote a typed CSS compiler at some point which was a lot of fun. I've used tools such as Lex&Yacc (Flex&Bison in C++) but I also wrote my own lexer at one point using finite state machines.
It'd be a lot of fun to get to work on a new or developing language again!