The game has up to Tier 5 POIs, but you can get +1 bonus for an Infestation quest for Tier 6. Are you advocating for 4 more POI Tiers or are you including the biome "skulls" in there, so +3 for Wasteland, or do you want to rescale existing POIs across more tiers, so the current tier 5 becomes tier 9?
Personally, making a Tier 5 POIs is already a large undertaking. I shudder to think of designing bigger/tougher POIs.
Instread, perhaps you're talking about a rescaling of existing POIs, existing Tier 5's would become Tier 9's and Infestation would be +1. If that's the case then I'm not sure how to show enough difference between the tiers, but I suppose that could be possible. You're likely to end up traveling more when doing trader quests.
The other option would be newer quest types that increase the difficulty, like Infestation did.
Which of those are you thinking?
By the way, the CompoPack may be your solace. They add I think two more Tiers to POIs to make room for their extreme challenge POIs, which would get you to Tier 7, or 8 with Infestation. Those POIs can come with performance issues because of their size and the number of zombies they've wanted to spawn at once, though they could be working to address that with Min Scripts. If you're looking for POIs that are trying their best to be nasty and kill you, that's them.