The CompoPack did go further in this area and defined exactly what the original post wanted. They're basically just bigger and bigger Tier 5's. The CompoPack has the luxury of not having to promise performance on lower-end computers.
Personally, I wrestle with the notion of Tier 6+. Perhaps I just lack the imagination necessary to envision what I would be trying to accomplish, plus taking into account what I think is possible in the game. Tier 5 POIs already can stress performance in their size and the number of zombies involved, specially when you allow for an Infestation (to get the current bump to Tier 6.) TFP uses minscripts to make waves of zombies and there are quirks with the minscripts.
As far as bigger and badder zombies, I tend to make the POIs flexible so that the game stage scales them. It is possible to ignore game stage in a POI and place specific zombies, but there's also an audience that doesn't like bullet-sponge zombies plus sandbox options to limit the high end. I don't know how that plays out with specifically placed zombies.
My own play involves placing more limits and constraints on myself to make the game harder, but I recognize that's not for everyone.
