[Rygar]

Anything that wasn't on the server prior to December 4th, 2001 is the answer.

I think some things can be reasonable custom content, particularly GM events such as Braknar's awesome Halloween event.

It would be cool to do things like that, but not confined to a single evening event. Perhaps there is a series of quests implemented for a 2 month span, no one knows what they are. Gather obscure 'no classic use' items from around Norrath (i.e. force neckbeards to use the other zones they aren't used to).

Rewards can be similar in line to the Halloween loot (removed items such as Rubicite, HEBs, etc) and some ToV loot.

Hell, maybe a custom quest chain for certain bottleneck epic pieces like Mage hate staff, white dragon scale, green dragon scale.

I would favor quests like this to just be extremely long, have multiple possible outcomes, and not be 'raid force' centered. Use your brain instead of your muscle to discover the rewards if you know what I mean.

Keep in mind these are just 'short term' implemented quests for a small time. They disappear, but perhaps re-appear a few years later but there are tweaks to what is needed to be found so you can't rush through it again and farm it.

Best part of the Halloween event is no one knew what to do initially (looking at you wiki), it actually recaptured that feeling of the unknown and rush of excitement from back in the day. Would love to see more of that.

Edit: Ooooh, forgot. Would be nice to use existing NPCs in game and existing drops. Make people run around and hail NPCs and try phrases like back in the day. Reward those that search