To each his own I guess. When Melee came out, the Adventure mode and classic modes were my most played modes. I loved running through the modes by myself.
I would love to see those modes come back with a vengeance. Subspace was wrong because they had stupid enemies nobody carried about and felt uninteresting. Adventure mode is simple. Create simple side scrolling levels based on each game universes world. Try to give each game universes world a different challenge with the games. Some games could have overlapping themes. For examples:
Mega Man, Metroid themes could be based on 2D sides scrolling shooting action. Characters could only jump and shoot. Limiting the moves of the characters can give variety to the levels. Mega Man levels only give characters one jump, but Metroid levels give you 2 jumps and focused on exploration.
Mario, Donkey Kong, and Kirby levels are based on platforming, and all attacks are gone, except jumping. Then you have to get power ups from those worlds. Kirby levels could have a triple jump mechanic adding to make them feel different.
F-Zero and Star Fox could be racing levels with Environmental hazards to deal with. Like the cars racing through...and you have to get on the correct path. This could create an interesting experience.
So you can see, an Adventure mode trying to capture the themes and play styles of the original games could be fun.