Remember that cropping can only be done chunkwise, so it will be impossible to follow a river like that. Anyway, here is my cropping proposal for the entire overworld. I tried to follow your plan around your regions.
Given that I think everyone is very excited about the new terrain, I have tried to be a bit stalinent?? (the opposite of leninent?) with the cropping, in other words, I have even cropped out some parts that I left there in previous cropping, such as northern Vatallus. I have marked some areas that I am unsure about in yellow, and feedback from people active in those areas would be useful.
1: Here I saw some circles on the map that were clearly player-made, I think in stone material. I guess stuffhappens might be the one responsible for them. I judged that they weren't important enough to preserve, so I plan to crop them, but I may be wrong, I could tp there I guess. If someone could inform/ask stuffhappens, that'd be useful, I don't really know any way I can contact him and he's rarely online.
2: This island had a ravine that I think Ventus, Mudkipz, possibly Darvince, Atomic and maybe Yqt explored long ago, and a camp in the ravine, but besides from that I don't think the island has anything special and that it has been abandoned for a long time, so I suggest that we just get rid of it now. If those who explored it want it to be preserved, say so.
3: Northern Vatallus. The decision of whether to preserve this area which includes a town is up to Mudkipz.
4: This area has the UBSR resource station I used as base for digging desert and stuff in October and the area includes the skyrail segments outside the red. Yqt wanted the skyrail demolished and my plan is to get on sometime soon and remove it so that we can just crop the area.
5: This desert, I built a road through it to the old Rossjungles and it had a village that I think is now dead... I liked the road a bit but it's not a big deal. It doesn't seem like anyone cares about the area anymore, anyway it's closest to Fiah so he should complain if he wants it to stay.
6: Quontex's settlement on the new quontinent he discovered long ago, this time I plan to just preserve the settlement and move it to near Quontia, since when he comes online I guess he'll be more excited about exploring post-1.7 terrain anyway. He probably won't have access to com to respond to this but if he really wants the entire island preserved I guess I could paste it back from a backup later.
7: Mystic Marsh I suggest be moved to the swamp near UYP and Mount Olympus, that Mudkipz built a road through. It seems like the most natural place to put it because of the biomes rather than having it spawn in the middle of whatever terrain will generate there after the crop.
8: Quontex's mushroom island I plan to move near Quontia, again because there probably will not even be an ocean around it after the crop.
So discuss. And I don't know how much time I'll have to crop it in the weekend, but I think I might have time. The math for next week is pretty easy and already mostly done, but on Saturday I have promised to help our local Unity List with putting up some election posters and spreading some election flyers around where I live, so Idk yet. The sooner people comment and agree, the earlier I can start working on the cropping.
Edit: Decided to shave some more land off Swamphold Swamp, down to the orange area instead. I think even more of it can be cropped actually, the ocean there is big, but I would have to be careful with the stronghold.