Yeah, “giant alien bosses in 3D” are one of the motivations behind this project.
I didn't know there were so many abandoned 2D to 3D projects. It's really unfortunate even though such projects are quite challenging. It's difficult enough to design effective 3D levels, let alone make sure the final product preserves the original's aesthetic. You are disassembling and reassembling these 2D classics in 3D, which made them legendary in their day. You have to make difficult choices during this process to get rid of some of the original components, like the jump flip that many players are attached to :) It will also introduce some new components, something like the sudden appearance of enemies that you mentioned. to make sure the game is still enjoyable.
I'm just doing this project for fun, and I'm not sure how far it will go. But I promise to at least create a huge alien boss :)
No matter what, I'm very grateful for your suggestions.