I'm curious how you'd feel about something like that to decrease higher tier weights
I'm not really sure. I mean everybody looks for something different in a map with different goals. Since A21.1, I've tried not to get in the way of that. I could certainly see somebody who didn't want preferred placement of the high tiers like TFP's default gives. I'm sure DF has unique needs. I say "rock on." I applaud your efforts.
In terms of bias and weight, my
interpretation is the TFP approach treads lightly on the numbers. I don't think they use weight (because it is a multiplier of score) and they use bias very little. I don't want to upstage or out-place the TFP POIs, so I don't mess with bias either (other than to test POIs). Their score formula has Tier/5 in it, so Tier 4 get 4/5 = 0.8 more score. That should give T5 the first crack at placement if there's a marker for it on the Tile being filled. If you want to level the tier, then I'm not sure how you do it other than to make a list of every POI and put a bunch of bias +0.2 kind of changes. I'd be curious to know how it turns out. (Go for it!) If the Tier 5s have to compete equally I suspect it might be more like A20, but I'm not really sure. The score formula wasn't know to me before A21 and I don't know if A20 used it.
In terms of DF, my
brief impression was that in order to get the story-unique POIs to place as desired bias was used with a heavy number, like 9. Other POIs got bias values like 2 and 3. That would get PEP POIs placed before Vanilla POIs, but a world usually has lots of spots to fill so I'm not sure I'd notice, but I didn't really try. I'm just guessing. I mean, if the DF players are happy then who am I to object? If they mix in my modlet, I won't have the same bias, so my stuff would compete like TFP's POIs, and if that's what's great for DF, then awesome.
Outside of a DF discussion, when somebody wants to mix POIs from multiple sources, things
can get complex for them. Some of us package our POIs in modlets and some do not. This confuses people and sometimes they think they have to take apart a modlet to install it. (Oh how I hope Steam Workshop integration make modlet installation seamless.)
Of the modlets, some influence map creation. If they have multiple modlets that influence map creation and then those modlets can fight over settlement settings. At that point the player really doesn't have many choices if they aren't used to being arm-deep into the rwgmixer.xml details.
This is why I quit putting map change config into my modlet, but I'm not suggesting everyone should do that. I just came to the conclusion that messing with the map was outside the scope of my goals. I could put map stuff into another modlet and leave my POIs in a more general modlet.
I see this is turning into a ramble. (Sorry.)
I don't ultimately know what most people want, though I do see lots of requests for contradictory or impossible things. For instance, people will ask for large maps with no duplicate POIs. Umm, impossible unless folks get cracking and create another 5,000+ POIs. (I'd hate to see the look on the CP Team's faces if somebody bumped up their load to something like 7,500 POIs.)
Some people complain there aren't enough T5 POIs. Some people hate maps that don't place at least one of each T5 POI.
I hope it's also clear I'm not picking on PEP. I wasn't sure if the OP was making a map for DF or Vanilla. I know PEP includes changes to world generation and bias and that other modlets don't. How best to integrate? I have no idea, but deleting a modlet's rwgmixer.xml file is sometimes a viable hatchet job for a player who doesn't get into rwgmixer.xml details.