I think it may be a good idea to add in these forms mostly for the sake of having as many different fighters as possible? I don't know how long you are planning to make the game, but it'd probably help with replayability if there were like 100 fighters, as opposed to 50.
Also, I don't know if you've seen the GBA hack Dragon Ball Z Team Training, which did something similar. One way they get around the lack of fighters was to put in random enemies that the DBZ people fought, which acted like zigzagoon or poochyena and operated mostly as fillers.
Also: it would be cool if you added a Saiyan Tail item which changed the forms of the Saiyans that equipped it. It could also operate as the "mega stone" that is required to mega evolve them into apes.
In terms of stats: have you thought of operating the game in a similar way to the game Dragon Ball Z Boudakai 3? Each fighter could gain points in things like hp, attack, technique, defense, ki etc. I'm not saying that you should include the points system; you could keep the usual pokemon style automatic leveling thing. But it would be cool if the number of areas that need to be increased were more diverse than the usual pokemon games.
Have you thought of adding Garlic Jnr as a character and his minions, as well as Android 13 and 18? It would also be insanely awesome to see Rose Goku (I think that's what its called), and the characters from that arc? Apparently trunks gets a new form in that arc too (without too much spoilers).
Final point (I promise). There's always been a big connection to things like anger etc. Maybe try using Darumaka's ability where their form changes once their HP goes down to a certain point? So, for example, Kid Gohan would enter some type of rage mode thing which would hugely raise his attack but would lower his defense.
Sorry for the long post. This games looks great so far, and I am really excited to see what you produce. Good luck!
The number of characters will be pretty high! I've already inserted more than 100 and a lot of interesting fighters are missing (like black Goku, Trunks with its new form...).
I've seen team training, but I've still to decide if I want to put the random enemies you said or not, I already know that they won't be used as soon as someone finds better fighter (i.e everyone else!), so they won't be so interesting....
Maybe I'll add a few of them for completion!
Regarding the Great Ape system, I've already written something for them! They work as they should without the need of an item.
I'll showcase the system very soon.
I'll tell you even more! I'm planning to include the Blutz Wave Generator... It'll be useful for the Golden Great Ape transformation...
Regarding the stats, the ev system of Pokémon games is pretty similar to what you've described, hp atk and def are present, special atk is the same as ki... It works well!
I'll even include things to train your characters like weighted clothings and similar things!
Yes, Garlic is planned, as well as Black Goku, Zamasu, Future Trunks...
Thanks for the spoil warning for those who haven't seen the latest episodes (I'm not included! I've seen everything! Can't wait to see Jiren in action! Eheh).
Regarding rage, effectively it could be a nice ability for some characters.
Thank you for your ideas, that's what I asked so don't worry about long posts! Eheh
If you want to say something else you're free to do so!