Essentially, the roof of the nether seems the most efficient place to build a farm, but you have to glitch yourself up there. A farm in the nether itself will require extensive slabbing of the surrounding areas. An overworld farm needs a lot of obsidian, depending on the number of portals you make. But apart from that all you need then is water flows to drop the pigmen down a central shaft where they fall to their death, and hoppers to collect the loot.
In the above thread myself and Sharp103 have been having a discussion (also a private convo as well), as he has been building a farm at his overworld castle, based on mine. Mine is a dual farm in that it can be AFK overnight to collect the pigmen drops, or it can be used in manual mode where I AFK for 20 mins or so the chop em up with a Looting III sword, both enhancing the drops and getting XP to recharge mending gear. But that requires more redstone and design. My farm is 18 23x23 portals (nearly 1600 obsidian), and Sharp's is the same; he recorded that he acquired approx. 1236 nuggets in 4 hours - that's just the fall to their death design. So that's an idea of what a farm my size gives you.
The AFK area is just under portals so there are no despawning issues.