This is just some ideas that I think should've happened within the Franchise prior to Super.
1. Super Saiyan 3 Vegeta: I would've had him do this in GT at some point, maybe during the Super 17 arc. It just seems unfair that he only gets it in video games.
2. King Cold in any kind of form: My own headcanon is that he's much weaker than Frieza and only has two forms: his true, or final form, and what we see him in. THis might not be the case, but I wish they would show something about this.
3. Max Power anyone besides Roshi: I think Roshi's Max Power form would be an ideal transformation for the human warriors. Super even shows that they could overcome the form's weaknesses with training.
4. "Good" Super Buu: I loved Super Buu and I hoped that Mr. Buu would resume the form in Super, but more benevolent.
5.Super Saiyan Bra/Bulla: Another GT one. She could've at least used this under Baby's control.
Vegeta ended up acquiring a mutated stage of SS2 that has all the power of Super Saiyan 3 without its drawbacks, essentially making it stronger than SS3, so he doesn't need SS3. Pan should have gotten Super Saiyan in GT as early as the Baby Saga. King Cold is shown in a restricted form and has at least his true form to go into, we just never get to see him do it. Fat Buu trains hard and essentially warps into Super Buu in Super, he just stops training and sleeps too long and loses the form. I also agree that Tien and Krillin should get the Max Power form and master it.
I don't know about the max power form for krillin and Tien... Imagine how odd that might look... I do think that they should get a transformation for themselves. I kinda get tired of seeing earthlings as helpless. Krillin could get so much stronger that way. Kaioken, Potential Uleashed, anything! Just make them stronger so we can get a reset on the Goku-everyone else rivalry. As for King Cold, I think he was in his final form, but it was almost as strong as Frieza. Just like how Cooler had another form but didn't seem to be that much stronger than Frieza in his 100% form. Vegeta should get SSJ3 at some point in time. I mean, come on! It may not be as strong as SSGSS, but I would love to see it canon-ized, or have Vegeta say "SSJ3? What, I mastered that years ago! I just prefer SSJ2." Pan should have gone SSJ, and still should. No telling how much older she'll e by the end of the NEXT series of Super, and Bulla too! I liked how Buu used the Super Buu thing, but he should remain fat. Super Buu should be a transformation for him to achieve. Maybe even the Kid Buu transformation too. All of this would be awesome to see in the future. Since we can't change the past, the future is all that we can rely on now. We also need Cell again. Imagine... Super Perfect Golden Cell (Which he can achieve since he is part Frieza) versus Super Saiyan 4 Gohan (who I'm hoping discovers the form along with Broly). See the two of them go head to head in a sort of Cell games 2. The Super Cell Games!