Hi,

This is my opinion, but I think the insertion of a "theme" conflicts (even indirectly) with the following point:

Can I add mechanics/content to the original game or can I only improve on what's there?

You can add, remove, update, change, and morph the original in any way you like. However, part of the judgment will be finding a connection between the original project and the new one, so there will need to be something in common between the two.

If I have an interesting concept (mechanics, etc.) that does not fit the proposed theme, force a "fit" in the revision may harm the connection with prototype or result in a very poor/generic/gratuitous sprites change.