Being an ex-GM myself, I have a unique perspective on things like this.
Scavenger hunts are fun to hold, but sometimes problems arise with language barriers. Meaning, players who do not speak fluent enough English sometimes have a problem understanding what specifically they need to do in order to participate, which is unfortunate.
I found that the event was more fun when you asked for several obscure items (from maps or areas that were far apart, in order to make the event last for a while) and maybe one semi-rare item.
Hide and seek/pvp/duel.
These were always a LOT of fun. I would hold something that was similar to a fusion of the three. Of course, I did this in Aion (asmodian faction vs. elyos).
For example, I would hop on my Elyos GM character, and go to the Asmodian lands and advertise an event for hide and seek pvp. I'd give out subtle hints at my location and tell players that not only did they have to find me, they'd have to kill me in order to claim their prize.
They really seemed to enjoy this, since often times players would team up to take me down. Of course I would split the rewards accordingly for the person(s) who found/killed me (and then i'd repeat this for the other faction, of course).
Q&A is...ok people like it because its simple, and easy to do. This type of event suffers from potential language barriers though.
Also used to do faction wars in special maps that players couldn't normally get to. That was always fun.
Anyway, to get to the point it's hard to find a balance where everyone can participate in an event, and the type of event you're holding. For example its hard for people new to the server to participate in pvp style events since their gear normally isn't up to par compared to others (unless they're heavy donators, which is cool).
Personally I liked to hold events that challenged the minds of players, and was still engaging gameplay wise (like pvp for example, or exploring).
As far as what I would like to see more of on here, I don't know. I haven't actually been in-game for a while due to horrible internet (last month) and now college classes picking up again.