More things to do would definitely top my list. I think opening entire lands with only 2-3 rides doesn't make sense, but once there's a solid 5+ rides, they don't all have to be "big" things. Somewhere in Harambe there are drums that kids can play - the kids in my family always get such a kick out of those. My son would spend all day at the trains in Epcot if I let him. He loves just sitting on the big fountain in Germany and walking along the edge of it. And bonus if the clock starts up while we're there! Things like that. I love the "little surprises around every corner" approach.
I would be a fan of having more interactive displays or interactive mini museums / "experiences".
Benches or chairs in the shade, but that's such a pipe dream I probably shouldn't bother typing it.
Greenery. Love the green spaces in Epcot and AK. HS could use more. MK doesn't have as much but is the one park that doesn't need it, IMO.
Things to climb on. But that is super specific to my child, lol. Seriously though, my complaint with Toy Story land is that while visually cool, the fun themed elements are all basically "Don't touch that! I said get off that!" kinda stuff. Not saying you can climb tons of stuff in World Showcase but there are fountains, bridges, etc.
Things that are unique to that land only. Food, drinks, merch, etc. They usually do a good job with this already.
And of course a visually stunning environment, that probably goes without saying.