Title says it all, how closely do we have to remake the game? Let's say I'm going to make Pokémon Emerald in GB style (not doing this, just an example), do I need to absolutely follow the GBA version, or can I throw some extra stuff in it?

I think it works as long as it sticks to the topic of the game itself: Pokémon

We have an faq channel in the Discord server linked on the jam page, it says:

You must follow certain restrictions, such as color palette, screen resolution etc. however it is more for the feel of the console. It must be distinctly portraying that consoles feel, but it does not have to stick to technical stuff such as storage space. It is mostly for the fun of it!

Hope this helps!