I know this response is a million years old... but you would cut the content that adds replay value to the game, new modes primarily, but would keep trophies, which literally do nothing for the value of play? Do go on. This seems a curious stance to take.
In a word, yes, but I'll definitely explain.
Trophies add more replay value than all of those things by the nature of requiring you do a little (or a lot) of everything in the game. They also represent the essence of Smash more than anything that doesn't take place in a battle - a showcase of Nintendo and gaming history. They can take you weeks or months to collect them all, providing more replay value than anything but Versus mode itself.
Customs and stage builder shouldn't require much explanation, but they both are things that you try and you either like them or you don't. Even many people who somewhat like them view them as gimmicky extras. They definitely have their dedicated fans, not saying they don't, but I could easily spend 20-30 hours more hunting trophies than I would tinkering with stage builder or custom moves and saying "that's cool" and sticking with vanilla options.
As far as "robust" single player, I mean things like SSE, Smash Tour (which is "robust multiplayer ig), excessive number of events, equipment, etc. I'm not suggesting to get rid of traditional Classic or All-Star modes or basic old school events like Home-Run. I would even love a well-done Adventure mode or SSE sequel, but Wii U had nothing of the sort but instead added a bunch of extra content a lot of people never used.
My main point was saying I would sacrifice all of the above "extra" stuff for several extra characters if it was either/or. The last "extra" thing I would cut would be trophies for the reasons stated, because I personally love them and have found more replay value from them than almost any other single player content combined. You obviously still need to have different modes to be used to unlock trophies, but again, I'm fine with a smaller Melee quantity of modes if they are full to capacity with content and rewards for doing them. A lot of the extras in Wii U had no incentive to do them if you didn't enjoy them other than maybe one trophy.
Also, I'm not a dev, but you could probably create several dozen trophies and spread them around the existing popular game modes in the time it takes to create one seldom used gameplay feature.
Just my preferences of course, hope it clears up my thought process for you.