What they really need to do, is go back to the yellow hair and make something look different about it.
If anyone, despite how tough it is, can reach UI, then it's not a Saiyan Transformation. Therefore, UI will NEVER be the last form because it's going to need the Super Saiyan stamp on it. Make it gold like SSJ1, add a little whatever to make it look different, some white streaks or something, call it a day.
The last form should be iconic, going back to the roots of SSJ1. And once Goku obtains this, you can't push it further.
There is no SSJ2 version or SSJ3 version. And that's what they can call: Saiyan Instinct.
I said it first. lol