It doesn't have to open the unit stats. Just selecting the unit, so you can give it orders, would be nice. That would be much more intuitive than having to click on the hex, then moving the mouse and clicking the unit's name one the list (which feels especially strange when there is only one unit to choose from).
And when there are several units on the hex, the whole stack becomes selected (or "activated").
When you dk on a hex that contains a complex but no unit
, then the complex details would appear (as UsF said).
Same for a hex that contains nothing (so you can build something without going through all the menus).
When there's a unit and
a complex present, only the unit gets selected.
I'm sure, there could be even more uses for doubleclicks. Most every software and game that I know of, applies this principe: When you click on something, something gets selected. When you doubleclick on something, either even more gets selected, or something happens. When you right click on something, either a menu, or general info comes up.
There's just too many instances in 2020, where clicking on stuff doesn't do anything at all. In the defense interface, for example, where all you units are listed by deployed and reserves, it should't be necessary to push a separate "add to selected units" button, just to select a unit (which, to make it even more cumbersome, makes the list disappear, and your having to click through the menu again). Simply clicking on the unit in the list should select it. I'm sure that's what 99% of all players would expect. Shift-clicking should add it to the pool of already selected units (it works this way in most other pieces of software).
The truth is that, while the interface has been immensely streamlined, compared to 2010, and it also does look very nice, many things that one would expect to be possible with single or double clicks, or by pushing a button that is readily accessible, in 2020 involve numerous mouse operations. In fact, the amount of scrolling through lists and menus necessary has actually increased, compared to 2010, because they all now have to fit in that small box.
The most striking example is the unit order list. This one, of all things, ideally shouldn't involve scrolling, at all. One possible solution to this would be to remove the name of the order and only leave the symbol. That way, there would be room to arrange the symbols in several parallel columns, instead of a single one. That would eliminate the scroll bar, while players would still be able to tell the symbols apart (other games use symbols, too). When in doubt, there could always appear tooltip, telling the symbol's function, when you mouse over it. Having a keyboard command for every unit order would do the same, without the need of redesigning the interface.
Finding a country in the diplomacy interface, is another one. The scrolling involved is simply immense, despite the regions being grouped by continent. Having the list expand vertically, so it fills the entire height of the screen (but not horizontally), would alleviate this greatly.
Same goes for the list of facilities. Presently, they're all lumped together, sorted not by type, but by complex. It's simply not useful like that. The icons are too small, the list too huge, the listing too obscure (if they were at least ordered by type, that would help a little). Bringing back the large facility overview window from 2010, with its tabs and nice large icons and fonts, that would be excellent.
Please excuse my long rant, but this pretty much sums up the total of my peeves with SR2020. I vented my steam now, and I promise, I won't be nagging any more in the future.