After 30 something years of going to WDW at least every other year and the last 3 years twice a year there are still so many things I haven't done. The last two trips I decided to do just as you said, either things I've never done or haven't done in many years. One in each park. Last trip I rode the Liberty Bell (never done it before) at MK, in Epcot saw the movie in France (never done it) and the one in China (been years), in AK did Birds in Flight (never done it), at HS I had a little problem finding something I either hadn't done or wanted to do but ended up getting something from Starbucks (I hate Starbucks so that was a stretch so I got something to eat). This Sept. I'm thinking of the following:
MK - either go see the Presidents (been at least 15 or 20 years) or Bear Jamboree (again been at least 25 years).
Epcot - See the movie in Canada or get one last ride on Universe of Energy before they close it down (been more years than I can count and it may already be closed when I get there).
AK - I may do Kali River Rapids (I've only done it once years ago since I don't like getting wet but what the heck maybe it will be raining) or find the trail around the Tree of Life (I know it's there but have never found it) or there is always ride EE at night or go to the new night show or do anything at night since it's never been open at night when I've been there.
HS - the problem park for me again. I may take some Bonine the night before and attempt Star Tours (those kind of rides make me sick) or do a meet and greet with Chewie (I'm not a meet and greet person but love me some Chewbaca) or in a pinch ride RNRC (haven't ridden it in at least 7 years because it tends to hurt my neck). If it's offered I might splurge for the dessert party, as of right now it's not offered when I'm going to be there. Or I might break down and do the Frozen Sing Along, I'm against anything Frozen since I think it's been way overhyped and I didn't like the movie that much but in the spirit of doing something I haven't done, I'll suck it up.