I voted for smash run.
I never played it but it looked like fun. It had plenty of brawl callbacks with the enemies too so there's that.
It has the most genuine experience of any other mode on the list and actually feels like something worth an update, content wise.
Though it'd never happen because this would be impossible to implement as paid dlc, due to the multiplayer/online experience it focuses on. It can't be a free update either because, who the hell wants to work for free/ less pay to create an extensive mode full of content? Game developers aren't slaves, so I don't expect them to work for next to nothing.
However, besides smash run, every other game mode on the list (including Melee break the targets) is honestly...
Unnecessary.
Melee break the targets isn't happening people. Sure it's a finite zone for each character and it doesn't have to be filled with "too much" detail but still. One for 70+ characters? A little bit of a reach there. They could most certainly do it, it's not "hard" it just takes time to make 70+ unique individual mini game target breaking stages. This also includes Board the Platforms.
Stage Builder is unneeded. In a game with over 103 stages (300+ with alpha and omega versions) where is there a need for it? I mean sure they could make one but I'd rather have a fleshed out stage builder than the lack luster and shallow experiences we've had before.
I suppose there are advocates for the Home run contest since it's been in since Melee and all but... punching the hell out of sandbag and sending him flying by perfecting your characters abilities and maximizing your damage out put by stringing the right moves together at the right time...it can feel satisfying but I honestly don't miss it. That was something to do when you're bored, yes, but if you're bored you should just go do something else instead.
Smash is a party game. Last I checked, anyway. Get some friends and beat the hell out of each other. The single player experiences can all be damned so long as the multiplayer core of the game has enough content (and it does).
Now that i'm done peeing in everybody's cheerios, feel free to disagree. Remember folks. "Opinions are like Mixtapes, No one wants to hear yours."
Mine included.