We put in the "significant element" requirement to avoid counting every church and Baptist mission - I had thought of the Anglican and Catholic Cathedrals in Zanzibar but didn't see them as that "significant" - though possibly they should be included here as they are mentioned by ICOMOS. Another issue with Zanzibar is that, although it fits under the 5% Christian "bar" we set, Tanzania as the State Party doesn't - indeed relatively few African countries are going to come under 5% (not even Egypt because of its Coptic history) other than the most Islamic (Libya/Algeria/Mauritania etc) but I don't know of any "significant" churches in inscribed areas there.
Another I had thought of was Istanbul - There are the remains of plenty of Byzantine churches but I don't know of any which are still operating within the inscribed area. There may well be more "modern" churches which are but are they "significant"?.
Then there is Baku (Azerbaijan has c 4.8% Christian +/-) - the main Armenian church there was attacked and damaged during the Nagorno Karaback war so isn't "operational" -but i don't beleive it is in the inscribed area anyway.
So, I am not sure that there are many more unless we increase the "5% rule" (It could be said to be illogical since we adopted a 1% rule for Buddhist temples - though I suspect we could up that without adding many more. The aim is to arrive at a figure which pulls out those countries where the presence of a "minority" religious site is of some interest/note