I have two premises...

So I'm completely and utterly new to ROM Hacking and the like (hell, even with DLing roms I'm still relatively new to the scene). In any case, I currently have two ideas that I think would be interesting. One is essentially a kaiju version of pokemon, while the other is much more fantasized.

Kaiju Idea

With this, all pokemon would essentially be replaced with kaiju from various franchises and such. For example, the starters could be Godzilla, Ultraman, and Gamera. The gym leaders could replaced by the various villains and mad scientists that each series holds. I don't know what...or more so who could be counted as a legendary, but I'm open to suggestions. I was also thinking the world would be much more war-torn and such because of the presence of such creatures, with some areas being ruled as a "Battle-Free" zone.

Fantasy

This one is a bit more far-fetched. With this idea, the world isn't within the Pokemon Universe. Instead of pokemon, there are demons. Sacrifice, bloodshed, worship are common place. You are simply a nameless wanderer who is thrown into the fray in-order to help bring the world to order. It would be very heavily themed by Pagan mythology and the concept of the Jewish hells (Tehom, Sheol, etc) which would act as in-game locations. Legendaries would probably consist of ancient, powerful beings who helped shape the world.


What are your thoughts on these? Are they too ambitious or doable with enough time and energy? Any suggestions?

It depends, if all you want to do is replace pokemon with new creatures, you could easily do a sideshow hack where the main storyline merely has some text changes, and all the pokemon are replaced. Sideshow showcase hacks are generally on the easier end of hacking if you are more a graphics or system updates loving person than a script/programming stories loving person. As a bonus, you will always have a complete storyline to completely play through if you simply borrow from the original scripts without changing them beyond text/images.