In the story mode, some characters are already transformed into their final forms (such as Buuhan and Super Perfect Cell). I was wondering if that would be possible in Vs. game modes, as it would add challenge at the start of the battle. It could work like the HP/KI selection bars, but for the character's forms.

Possible, but very complicated to add now. Furthermore, transforming ingame takes seconds.
But I'll see about that when Dragon Ball Devolution will work using Ruffle, in 2021.

To add onto this, I believe a system like BASTON would work best for the character selection, giving the ability to customize each characters HP and KI levels. As for the "selecting transformations", I agree that it only takes seconds and therefor isn't totally needed. What I think IS needed is a way to LIMIT transformations, so we can actually fight against First Form Cell, or Second Form Frieza, without them transforming.

My idea is that in the "Character select" menu, once you select a character with transformations, it would bring you to a sub-menu which displayed all their transformations, and clicking on them will grey them out, which disables them. Here's a quick graphic I threw together to demonstrate:


In this example, Goku would START the fight as Super Saiyan God. Transforming once would make him go Super Saiyan Blue. Transforming again would skip the next two and jump straight to Ultra Instinct Omen. Goku would not be able to transform into Mastered Ultra Instinct.

For things like time-based or health-based transformations, they wouldn't need to function any differently, simply skip to the next transformation in line (or, if they're transforming down, go to the lowest allowed transformation.)

Finally, I just want to say, I fully understand that this would be very hard to code, what I'm saying is not at all commenting on the complexity of the programming. This is just a formatting idea to make it the concept appealing to the player. As just a concept, I think it's pretty intuitive and easy-to-use, as long as it has maybe one sentence of text above it explaining how it works.

That's a very interesting concept, I think it's better than my idea to be honest

I'm currently trying to figure out how I'm going to handle that in programming language.

The simple solution would be to be able to select start form. And even that would break the game in the case of absorbing Ki and transforming with Android 19 or Yakon.

Then, if I got this to work, we could select the maximum transformation a fighter can reach. That's the real easy part.

Cairns proposition is interesting, but it raises many problems with the way transformations are currently handled in the game... Selecting only some transformations would mean skipping some forms. This will complicate the simple "current form + 1" when powering up, and break the powers such as "levelDown_2" to get back to second form while powering down, instead of doing "current form - 1".

Lastly, some people will take ages to select their fighters in Versus mode (or worst, online mode). And that's really counterintuitive and not fun at all...

So I really need time to think about this.