Here are some additional opinions:
While he doesn’t seem that interesting to me as an SSB fighter despite being my favorite Mario character, I’m shocked that Toad isn’t highly requested since the SSB fanbase seems to emphasize hype and legacy, which are two things that Toad has (I’m confident that a lot of casual fans would like his inclusion).
Relating to the roster, there isn’t a strict formula to character additions. Some are from legacy like Bowser, Wario, Villager, others are relevancy/promotion with Lucario, Ike, Greninja, Byleth, they can be requests like Bowser and Wario again, Little Mac, Ridley, K. Rool, some are easy to make like Jigglypuff, Roy, Wolf, Richter, or they are just fun to play as like Ice Climbers, Sheik, and Robin.
I think fan rules can be a useful way to keep some expectations in moderation (eg not expecting the last DLC fighters all being AT promotions), but they aren’t super strict rules that must be followed (eg Sephiroth being the second unique fighter for a third party franchise).