I'd say Sakurai has been quietly working on something for the past few years. Based on Brawl, Kid Icarus: Uprising, SSB4 3ds/Wii U and DLC Sakurai has shown that he will go above and beyond in terms of content and effort. I think if he is just doing a port of Smash 4 then he is still going to add a lot of content to entice players to come back. Here are the options I'd like to see:
SSB4 Deluxe- Switch port of all 4 content from both versions with 5-10 new characters, New stages, fleshed out Build you own stage mode, updated character models and New single player adventure mode.
SSB5- altered and enhanced Smash Bros 4 engine and character roster/moveset. Game launches with 40 characters and 40 stages with a Splatoon/Arms free DLC release schedule. Stages are 60% returning stages from previous entries. After a year and a half of free DLC the roster ends up being 70 characters and 80 stages.
SSB: Legacy Tour- enhanced ports of all four entries' versus mode and engine into one game. Game starts up the first menu allows you to either pick Fight, Smash, Melee, or Brawl. Each mode uses the same models and assets, the difference is that each mode is a recreation of the versus mode from their respective title. The character roster of each mode includes some new characters that weren't in the original. For example, Fight mode has 4's full roster with Ice Climbers, Wolf, Snake, Inkling, Spring Man, and Bomberman being newcomers to 4. Fight mode also has 4's All Star Mode, Smash Tour/Run, and all of the new, original stages from 4. Smash Mode would have 64's roster with Bowser, Peach, Marth, King Dedede, Mewtwo, Little Mac and Pit added in. You can choose from 6 64 stages(4 of which were in 4) and 15 other stages from the Smash bros series that are present in other modes. Smash mode also includes 64's classic mode, Break the targets and Home Run Contest. Melee mode is as faithful of a recreation as possible to the original. Roster is mostly the same(no diddy kong) with Diddy Kong, Wario, King Dedede, Metaknight, Little Mac, Lucas, R.O.B., Sonic, Pacman and Megaman added to the roster. Melee mode also has an enhanced version of its Adventure mode, Board the Platforms and Race to the Finish. There are 16 Melee stages(Including the 7 that were in Smash 4) on top of 20 other stages from the series. Brawl mode has every character from Brawl except for Pokemon Trainer but it also adds Megaman, Pac man, Mewtwo, Charizard, Bowser Jr., Rosalina, Dr. Mario, Villager, Wii Fit Trainer, Little Mac, Ryu and Cloud. Brawl mode has 28 Brawl stages and 25 other stages from the series. Brawl mode includes enhanced Event Mode and Build your own stage mode. All Versus modes can be played online.