A few things can create a PP map
High object density. More stuff=more acc PP + More marathon bonus. If possible, more circles, as they add the most acc PP. A good example of a map with very high object density and circle count giving a high amount of PP would be
Frame of Mind. These maps usually give more PP than equivalent maps of the same difficulty.
Short but intense difficulty spikes. Famous examples of this include
Miraizu and both
Best friendS maps, and
Stella-Rium. The difficulty spikes increase the star rating significantly while the rest of the map is easier than the SR would indicate. As a result, the player only needs to "Luck out" the difficult parts to get an FC on the map, and reap the rewards as if the rest of the map was as difficult as the spikes.
Low BPM maps with simplistic jumps create great farm maps at both the intended difficulty level and when playing DT. Examples being
Setting Sail, Coming Home,
Koigokoro, and of course, the famous
Daidai Genome. Note that both Setting Sail and Daidai Genome have the advantage of OD8, which becomes 9.67 after DT, giving out lots of extra PP. Koigokoro compensates for OD7 with its shortstreams adding lots of acc PP from high object count.
A map with high OD giving large acc bonuses. A good example being
The Pink Fields. High amounts of PP given compared to equivalent maps of a lower OD, due to the way acc works.
Very short maps with a high difficulty rating (relative to the player's rank) that can, due to their brevity, be Full Comboed by people of a lower rank than one would expect, due to the low consistency required. Good examples include
Felt - In my room, and
Arcien - Future Son.
Edit: I forgot one more type. Certain maps have all of their star rating come from simple streams. This includes
d.m.c and
Rog-Unlimitation. If the player has the speed required, these maps are also an easy source of PP.
...phew, I think I went over all the main types of farm maps. Thanks for making this topic, it was very thought-provoking. Hopefully you've gotten your answer!