Open world was a mistake. MEA lacks the subtle small things

That really depends on how it's done. there are games that do it very well and I'd say, you being the Pathfinder and all, ME:A is one of the better examples. Sure, the boundaries of the worlds are stupid but there would have been better ways to do it (e.g. on Eos, instead of holographic walls, you just make the boundaries by having extreme radiation hazards beyond, etc.).

But in terms of story, I think it worked. What I was missing in ME:A though was the diversity. Basically, they made one planet and then copy/pasted the formula for it 4 times and changed some textures and effects. That's not cool for exploring (ME1 did the same, possibly even more so btw). There were only 4 places that were different:
- The Loyalty missions and the story missions, which were linear but you went into them on purpose
- The hub areasa, which were hub areas, we knew that much
- Hawarl, which IMO was a nice change of pace
- The asteroid that used to be the turian planet - finally, something a bit different with those domes and the different gravity and such, good stuff.

That's a start but it still lacks diversity because with the exception of Hawarl and that asteroid, you always know in advance what you are getting into. After Eos, you know, ok, this will be the next big Nomad planet or ok, this will be a linear loyalty/story mission, etc. and then you go and tick all the boxes for the pre-established mechanics for that. That's not cool for exploring.

What I want to see in Mass Effect game is the kind diversity where you as a player fly into a new star system and you do not know what you'll get. You actually have to go there and find out. It might be:
- A bigger planet like Eos (maybe not quite as big, 1/3 of the size would be enough)
- A very small mission like an ME2 N7 mission
- A nice, story heavy linear side mission like ME3's Gellix or Grissom Academy
- A smaller open area to explor with the Nomad to find some specific stuff (like the ME1 planets)
- An open area that has a story to it (like BDtS)
- A linear section for your the Nomad (like the first part of Virmire or the Volcano mapin Overlord)
- A non-combat more puzzle and/or dialogue based mission (like Samara's LM in ME2 or - very crude example but IIRC the only one ME ever had - that shield generator N7 mission in ME2, puzzles could be expanded on with the scanner and everything, see here)
- Maybe, just something dialogue based, where we stay on the Tempest and  communicate with another vessel (hasn't really been done in ME for an unknown reason, happens in every single Star Trek episode though)
 
The possibilities are almost endless. ME already has this natural division of their game world, where your ship is your mobile hub and you access maps or levels from there at your leisure. It's a great setup to introduce diverse gameplay that will never get boring but what gets me in all the ME games is that - while the devs changed things around massively between games - they stay very formulaic within each game. Despite the fact that it would be so easy to implement this kind of variety through the different levels. I guarantee, if they went away from their formula and managed to surprise us and keep us guessing what we might find in each new star system we explore, it would keep players so busy and engaged, they could get away with a lot more shortcuts in the details of things (not that I want to advertise for those, I'm just saying, if something's got to give, it shouldn't be basic design variety).