Personally, I think it would be fun to keep the minimum requirements to bread, cheese, butter/mayo and a grillled/heat aspect to crisp the bread.

Multiple types of cheese, sure. Pesto, cilantro, dipping in tomato soup, cream cheese, all good.

However, I do think the main focus of the meal should be the cheese. If bacon is a accessory for the cheese like one strip then fine. If it is the main point (like 10 slices), then no. Same for chicken, burger meat, turkey slices etc. if you want to be creative and add one slice of bacon, one slice of Turkey, one slice of chicken, then you are overriding the glory of the cheese.

The golden rule with a grilled cheese is that you must respect the cheese. Extra ingredients can be included but if they detract materially from the glory of the cheese, you have made a melt.