I am going to mention many locations here which were relevant to me because I am sure others would post things for them as well.
Some things I worked on in Horizons would be the town of Mystic Vale located south of spawn at Z 700 and then spread out in each direction onwards. However these places aren't decorated or cleaned up. But some interesting points there would be Halloffameboy's house (X: 0, Z:750), the even zone currently consistent of one big tree which might be turned into a christmas tree again (X: -100, Z:830), the PvP arena (X: -50, Z: 950) and if we can get it on before then the Iron Golem farm (X: -200, Z: 800) which should generate 2000+ iron per hour I believe it was.
For other and possibly better locations around Horizons would I point you towards Stygian's place which has a fully automated for everything (Every mob let it be overworld, nether or end and many blocks) encased at X: 400, Z: 1400 but be careful going to there.
Another interesting place would be Shadow's puzzles and specifically Mount Shadoe in my opinion at X: -500 Z 1230 and further beyound in the south. Another Shadow build puzzle is in the North East which is the Horse Race (X: 1350, Z: -1430).
Other big locations which stand out would be Richmond by Furdabip (X: -40, Z: -500 and the whole island). The flying library by Tulonsae (X: 300, Z: -450). And I can't remember who build it again but it was a nice build around X: -800, Z: -333. And finally some underwater domes as a change of pace at X: -400, Z -1100 by Ifiwy
I am certain I forgot many things and that the coördinates won't be at the right places as I got them from the online map. But I am certain others would post below me with fixes or better locations. All the locations mentioned here will be posted below again.
Locations:
- Spawn (Never Forget): X:0, Z:0
- Mystic Vale: X:0, Z:700
- Halloffameboy's house: X:0, Z:750
- Event Zone (Currently occupied by tree): X: -100, Z: 830
- PvP arena: X: -50, Z: 950
- Iron Golem Farm: X: -200, Z: 800
- Stygian's Statue: X: 140, Z: 710
- Stygian's place: X: 400, Z: 1400
- Mount Shadoe: X: -500, Z: 1230
- Horse Race Track: X: 1350, Z: -1430
- Richmond: X: -40, Z: -500
- Library: X: -300, Z: -450
- Another build: X: -800, Z: -333
- Underwater domes: X: -400, Z: -1100