I already edited the specials myself by now. So…yeah!
Now, lets see…I’d keep the following as forced:
Refugee Center
Ranch
Laboratory (At least 1)
Bee Hive (1 forced)
Anthill (1 forced)
Triffid Specials
Fungal Specials
hell spire (PK)
sky anomaly (PK)
Curious Structure (Arcana)
Strange Grove (Arcana)
Floating Temple (Arcana)
Island Temple (Arcana)
Survivor Holdout (Cata++)
Bio Weapon Lab (Cata++)
Unknown Lab (Cata++)
Makeshift Command Centre (Cata++)
Basically, forcing at least those will make sure that most enemies and so on will be able to definitely spawn in your world. The Hell Castle is, by default, not forced, so it should probably stay that way, after all, you will be able to find it eventually by driving through the country.
Otherwise, some of those are also necessary for ‘questlines’ and similar stuff, so you would want to keep them forced for the player to be able to do without driving for several days. Of course, you have to consider that basic stuff like Schools, prisons, Mansions and so on should spawn semi-regularly, so you might want to make them at least partially forced too. Otherwise, if you do it right, you can make sure to stay under the 72 limit ; and every ‘spot’ you don’t force will be, more or less, completely random.
Oh, just to be sure you don’t make any mistakes:
"occurrences" : [x, y],
that’s the line usually looks. the first number, in this case the X, is the >Minimum< amount of said special. This is the number that is forcing the game to generate a special. The second number, the Y, on the other hand seems to be the maximum amount of a certain special that can spawn. So, if the numbers are [1, 7] that means this specific special is forced to spawn at least once, but could spawn up to 7 times in total. This is important for ‘simple’ specials, like ponds, mansions and so on - those should have a higher maximum amount, so they could potentially spawn more often than not. Stuff like the hell castle on the other hand should keep low numbers, to make it reasonably rare. You don’t want to have one in every town after all.
In general, places with high-end loot or with a lot of loot should be more rare than places with less loot or only very simple loot.
This can also be indirectly be used to control the amount of survivors, if you have a fitting mod. For example, there is a merchant mod out there, made by “Undereastern” which adds several overmap specials, mostly small survivor encampments or lone traders in small bunkers. You could, in theory, use those specials to make survivors a little bit more common, if you raise both numbers. Or you could keep them at 0-1, in that case the player would have to search around in order to find small enclaves of other survivors. Both could be interesting scenarios.