This has probably been suggested before, but as im not sure I will post it anyway since I found it to be interesting.
The whole idea is that when you are awake you use your imagination to make up a few mythical “ingredients” to create a Lucid Elixir. Also make sure to connect these ingredients with different places like a herb that grows high up on a glacier or a fish that lives deep in the ocean. Once you have your list, decide what effect this elixir will have in your dreams and write it down too. This can be everything from flying to invisibility etc. Of course you could just use dream control to reach those results, but by doing it like this maybe you can trick your brain to the same effect while having a bit of adventure? Make up a few recipies for different elixirs and try to memorise them somehow so that you can remember them somewhat for later.
Now for the interesting part. Hunting these ingredients in your dreams. Maybe you can collect all to make that “Dream prolonging elixir” you thought of IRL or just a few of the ingredients. When you wake up your note your progress in your recipies. My thoughts is that you get some adventure while getting these ingredients. And when you turn them into a elixir, maybe you could use them to achieve effects in your lucid dreams? Im going to start working on this idea in my own dreams.