I'd say it goes with interest. Last year we added all the bonuses stuff (individual btt / btp, all btt, all btp, all bonuses).
If there's enough demand for a category, I don't see why we couldn't add it, but we wouldn't add a category for 1 person either.
There was a meme category I did in the past called roster challenge which was beat all 12 characters LVL 9 with i believe only 4 stocks for the entire run. Might've been 5. (eg you lose 1 stock to luigi then you only have 3 for the rest of the characters). It was done on Dream Land (of course) and no items. I thought it was kind of a funny meme to try out. I always tried to do it in every smash game but it would get tougher because I would normally try to follow the stock count permitted in a crew battle of sorts or how many are allowed in a competitive game (ie 4 stocks is used for melee in competitive so that means you have 4 stocks for the entire roster challenge. Or in brawl you only have 3.) Which means the newer the smash game, the more challenging the run got rofl.
As for other difficulties, I was doing some runs of Easy 4 stock & eventually Hard 2 stock runs. I think the categories are a little interesting and different enough from very easy and normal 3 stock that they could make legitimate categories. In terms of seriousness and legitimacy I think adding these two difficulties would be something to consider making for the boards. But only if they can get enough interest and other people would agree to it
As someone responsible for many of the meme categories in the history of this game:
Be The Meme You Want To See In The World!
Don't run a meme category to get it on this leaderboard. Don't run it with the expectation that others will try out your category; If you have passion for a category and it's a good category other people might try it out.
Make your run the best you can regardless of category restrictions, and be satisfied by the competition you can offer yourself. There are so many categories possible in this game, if you find something you really enjoy and think you want to be more optimized then treasure it and cut that time down.
If you wanna theorize janky categories you will find ssb64 is vastly more open to this kind of theory-crafting than most communities, everyone at the top of both 1player and bonus are extremely open to this kind of discussion. Take advantage of the curiosity of our playerbase, get on the discord, and ask questions about how to optimize your run. Where most speedrun communities have hard boundaries around category interest the largely transferable strategies between categories mean just about every top player is extremely capable of theory-crafting regardless of activity!