I'm sure there are many new ideas from slot developers but I believe the reason why they keep on using the same type with different theme is about the interest from users/gamblers.
If a specific type lets say "Megaways", has so many players then developers will use the situation to create more similar slot but with different themes only.
I was someone who love to try new slot games in the past but I did not do it again since few months ago.
For now, I prefer to play my favorite slot only instead of trying new slots so I have no idea if there is a new innovative slot game released lately.