I think i know what's going on with the leagues system. Standard and hardcore are always there as your permanent options, while challenge leagues pop up every few months with fresh starts and new mechanics. Starting in standard lets you trade and team up freely, plus you'll keep everything you've earned over time which is nice for learning the ropes. The interesting part about hardcore is that dying just moves your character to standard instead of deleting them completely, and you can make things even tougher by picking solo self-found which stops trading and partying.

Challenge leagues are where most players hang out since everything resets and you get brand new content to explore. When these leagues finish up, your stuff moves over to standard which keeps building up the economy there. Maybe interesting to know that you can't manually switch between leagues except when your hardcore character dies and drops into standard.

Solo self-found is pretty flexible since you can add it to any league if you want to challenge yourself with no trading or teaming up. Most players jump into challenge leagues for the fresh economy and new seasonal mechanics, but standard is great if you want to keep building your character long-term without restarting.