To actually submit a worthwhile post to this discussion, the leak actually gives me some hope in this regard. Remnant vaults sounds like it could provide a good gameplay challenge (just do it better than Destiny's horde mode). ME2 constantly added extra elements to its fights that made each one unique. If you do Samara's mission then if you are a biotic you can use the red sand to your advantage, but if you weren't it was about choosing your cover carefully to not be poisoned. Tali's recruitment mission made it so you had to avoid sunlight if you wanted your shields up. They never felt like gimmicks either because they weren't the focus, they were a supplementary challenge blended superbly well into the main task at hand. I think each Remnant vault could work the same way, each with their own unique challenge and each one upping the difficulty with new enemy types.
For the Khet outposts, if they indeed are our main foe in this game, each outpost should be handled the same way previous ME games handled their side content (mainly the loyalty missions in ME2 and missions like the Grissom Academy mission in ME3). Each outpost has it's own unique layout, each one with a boss who gets cutscenes portraying his or her character, including interaction with our MCs to make it clear neither one is good or evil, we each just have our own needs to worry about. Avoid the planet of hats trope by making each Khet boss unique as well, with their own beliefs. Each band of Khet that guard each outpost should also have their own personality and culture. These missions could also be connected by smaller objectives. In order to properly raid a Khet outpost you have to survey the landscape to find points of interest, you have to take out smaller camps of Khet scattered all over the planet. Maybe if you're an infiltrator you get a special mission where you can actually infiltrate the Khet outpost, using you cloak strategically, to do recon.
Then you can have the typical resource scanning, and finding places for where colonies are viable. It's a balance between eliminating the Khet, and finding a new home for your kind. All of the side missions you do should connect together for one grand goal so that when you do everything on a planet it all felt like it had a purpose. I also wouldn't mind it if there were optional quests pertaining to the different lifeforms like the flying creature in the trailer. Imagine a mission where you have to tail the flying creature in the Mako without getting its attention so it leads you to its lair. Then you wait for it to leave the lair so that you can kill all of its smaller spawns in battle and destroy the nest (if you don't then potentially another flying creature could spawn after a certain amount of time if you kill the first one). Then it's about actually killing the thing if you feel it necessary. Each "surveillance" mission on the monster will reveal its hunting patterns (ie reveal what kind of attacks it'll perform on you when you face it down) as well as make the creature feel like a living thing. I'd love it if planets could actually feel like an active ecosystem. Being able to witness one alien life form hunting down a smaller life form would be cool. All of this could feel like an extension of the dragons in Inquisition, and the monster hunting quests in Witcher 3.
The problem with Inquisition wasn't the world design imo, it was the fact that none of the side missions outside companion quests felt connected. You're doing pointless busywork like planting flowers at a grave or finding a ring which have absolutely no impact on the fate of the Inquisition. In reality the Inquisitor should be back home in their fort ordering forces around or going on missions of extremely high importance (like all the fantastic story missions, or the capturing of enemy keeps Bioware for some reason decided to dial down). Inquisition needed to keep the keeps as important and consistent side content with weight on the fate of the Inquisition. It needed more missions like freeing those fortes from the undead. More quests like freeing Inquisition soldiers from the Avaar. Those were great, but too few. Being the pathfinder, and not a figurehead, in ME:A is the first step in the right direction at least.
I think my expectations are a little too much though. I really just hope there is a solid reason that I can feel while playing the game, for why they decided to bring back exploration. If it's implemented like ME1 or Inquisition I think we might be in trouble (although the story and characters would still rescue the game from my condemnation. I still love Inquisition to death because I know how to properly limit the side content I do, but I shouldn't have to plan a route like I'm a speedrunner to avoid that stuff).