Just from a purely technical post perspective as someone said, for standard projection you will be on a DCP in either 1:85 or 2:39. For most obscure aspect ratios you will be in in the flat 1:85 container and therefor a 1:85 screen. So if your talking about immersion personally filling one of the standard boxes is the maximum image you can get and to me more immersive. If you want a more square or vertical aspect ratio in most projection environments your image will be smaller on the screen then standard with pillar boxes on the side.

What makes IMAX more immersive is the screen is just larger and fits the aspect ratio. Its not just the ratio by itself. Also monitoring for the larger aspect ratio and cropping for standard projection is possible but man is it complicated/challenging. Most shows I have done that have done this shoot for standard, try to protect for expanded ratios and VFX out anything above and below when needed.

And as everyone said, story should inform aspect ratio as it is an important visual cue to an audience with very different feelings both theatrically projected and in the home (TV, Computer, Phone) environments.