I'm curious to hear other's thoughts on they think would be useful.
Please don't hold back ideas! There's very little harm positing silly or unlikely ideas, but there is some potential value (perhaps more than you realize).
Inspired in part by calebo's comment here
Note that it is possible to do longitudinal analysis with the EA Survey, and we have done some in the past (such as for retention, GWWC pledge keeping, and changes in cause preferences). I'd be happy to help walk people through how they can do their own longitudinal analyses and what the relevant caveats are.