I wish there were more shows and maybe some other types of performers.
My wife and I love all the shows at Lagoon (except for hacknslash, that one is just irritating noise). The mariner one a couple years back was fire, the adopted sister one was great, my family always likes Smoke & Mirrors, this year's Z Emerald Underground is a lot of fun, the Wonderland one was great. Seance is my favorite attraction at Lagoon, even over all the rides.
I wish there were street performers (kind of like the marching band back in the day, but wouldn't need to be so elaborate). My son and I went to the new Universal Epic Universe park about a year ago and the storyteller we watched in Dark Universe was great.
Also I think a rotating schedule of local cover bands at either the opera house or the outdoor stage would be a ton of fun. It'd be a good way to spend some time when you don't want to wait in ride lines.