I'm afraid this has a shot at being the last time I post on this very thread so I'll sum up every thought I can possible and with respect.
1. Bringing in Kakarot (DB Goku) would only be celebrating the manga and anime. His games are popular for the very reason of recreating moments from the manga and anime or see new fights that would never be on source material. Were this a video game by origin franchise, we wouldn't have this point brought up and I wouldn't have objections. If the Smash team's vision is to celebrate video games strictly, Kakarot doesn't make sense. If this was a celebration of Japan as a whole, this point is moot and I wouldn't object to this idea. Problem is we don't know the Smash devs' vision for Smash at times. That's one point of speculation, because we want to know.
1.5. Let's stipulate (or a special agreement to make a point) that Smash is indeed video game focused. If I were hypothetically a game developer who just started allowing guest properties to show up in my IPs, I would personally announce my intention for said guest properties without ambiguity. Keeping with the hypothetical, let's say I wanted any video game by origin character from Capcom. I would not consider Cloud Strife, Batman, or even Mario and I would ignore these requests. I would listen to those who'd give me video game by origin characters owned by Capcom like Street Fighter's Ryu, Dante from Devil May Cry, or even go for something obscure. I could in this situation have some sympathy towards the Smash devs who get requests for things running counter to their vision in spite this point being a fallacy of author appeal. I believed it's justified by the creator of the work having the major say in what comes into a work being a factor. Breaking from hypothetical, this point merely an addition to point 1.
2. I believe it was stated that getting characters (both guest and non-guest characters) was really difficult this time around. Licensing and budgeting does play play a factor in relation to characters (as well as the the rest of the game). It's theoretically much cheaper to go for a guest character with only 1 IP holder entity than say 4-6 companies that own a guest character IP. This does take in the account of voice acting and other financial stresses involved. Kakarot is has Bird Studios(Toriyama's company) and Shueisha for the source IP rights while the Toei Animation holds the anime rights to DB. Bandai Namco currently has the video game IP rights to DB. The financial costs would be massive for DB and has a shot at draining the budget. I personally would avoid DB for this reason.
3. This is merely a collection of small, quick points. No character deserves anything in Smash or needs to be in, but they are only wanted. Ultimately, this is a video game and the best way to make a statement about this is to either get the game or don't. Treating popularity as the only factor is a mistake and there's more to Smash characters than popularity. There's moveset potential, possible uniqueness, demand, and reasonable limitations that comes with mere popularity. I don't hate the DB franchise, but I really like it.
4. This thread is dedicated to Yuyuki Goku, a Nintendo character I have some interest in. I'd like to see how Smash would use this character if he were to get in. I'd like to see more discussion on Yuyuki Goku himself and what he could bring to the table, here. Interactions, costumes, trivia, and moveseet ideas. Sadly, I only see arguments get repeated and it's not really fun for me on this thread.