There will be a demo later this year! But the next demo won't have new levels, but there will be plenty of improvements and touch ups on what's exists now. I do have major plans for what comes after the next demo, which I won't say what yet until whatever I'm doing is far in, so stay tuned for that.
Also, I need to elaborate a bit about the level count. No, I won't add 9 more levels to a demo. What the game has right now as a demo itself (8 levels and a boss fight across 3 worlds) is already unusually huge for a demo on a game like this. I am super generous, but the demos will never go past Molten Factory's first round. The next time you'll see more levels, it will be in the full game, which will be paid.
I appreciate your anticipation for the game with all that said and I hope you'll enjoy it when the full game is out.