Maybe this can be a common collection thread, unless as said I've missed one already existing. It would probably be best to limit suggestions to things you feel can be fixed without completely overhauling the UI (IIRC Vic has said he doesn't want to start an overhaul), and include a clear suggestion for fixing the problem if you can.
Starting with a big three that I've seen every single new player stumble on:
Resource display not warning when you ran out of a resource

Player will look at this and go "I'm gaining oil every turn, why aren't my tanks getting fuel?"
It should be something like this:


(for colour clarity the red should probably be much lighter shade though)
(also "Had" should of course be 959 in the example)
This will help the player realise that the SHQ resources are for next turn's distribution, not surplus after this turn's distribution. It's hard to overstate how confusing this is for everyone at first.
The "supplies that the units actually needed" information is already available, hidden under STATS -> Logistical Statistics -> Unit <resource> Supply that few know about, so it should be retrievable for this mouseover.
Similarly it should of course have the warning if assets were the ones which hogged the resource (data available in same menu under Loc <resource> Pickup).
Raise Formation should default to Brigade size instead of Corps

As said in Edmon's tutorial ages ago, the first thought of a player who manages to find the Create Formation menu is likely to be: "Oh, huh, the percentages on all these options are way too low for me, I can't recruit anything here yet". A lot of this could be remedied by simply having the default be on Brigade/Battalion size, which is the relevant option for most of the game.

Similarly the Multi Unit option could well default to Independent, since especially now that IP costs have increased on many planets it's unlikely the player regime will reach the 50% threshold required for raising an OHQ formation for several turns.
(Latter carries a slight risk of the new player not learning about OHQ formations at this point, but I think lessening the confusion outweighs it. There could instead be a tutorial message about OHQ formations. Unless one already exists. Is there some debug table where you can view all tutorial messages?)
Linear techs need more clarity in-game
Probably dozens of times I've taken a look at a new player's save (or seen their techs with spies in MP), and observed a single linear tech at 80+% that's been under research for 40 turns, with the player unaware of how they work. "I thought the Council will say when I can change it."
I'd suggest:
- Clearly differentiating them in-game, much rather going overboard than not making clear they're different from binary techs (maybe with more font / colour differentiation between message parts than in this example):
- Have the Director ask for new target every 4-6 turns if they're researching a linear tech
- Maybe even a tutorial message on top of it when the first linear tech is discovered (if they can be triggered in such a way).