* A base Leader level + Leader scaling somehow linked with Empire size
* Significant reductions to the penalty given for going over the Leader cap
* Techs which give increased Leader cap
So unfortunately all of these have a distinct problem in that, if I am reading these dev diaries correctly, they are NOT what the developers intend. They seem to want leaders to be a relatively fixed small number of characters at a time as a "get attached and remember these ones" type of thing. ANYTHING that, in a hypothetically infinite universe, allows infinite leader cap, is "against their vision".
Personally, I think that's perfectly fine! But then they can't have "planetary governors" or "every fleet needs their own admiral" or "every science ship needs their own scientist".
The solution that preserves the developer's vision and solves many of the community's problems is to eliminate all leader roles that are finite in their effects. This means remove scientists from science ships and have all of them controlled by a "field agent". Make all fleets controlled by 3 admirals, who confer their bonuses to "Screen ships, Battle ships, and Megaships" respectively. Eliminate governors as a sector level and have three positions that apply to separate categories of the industry, but require you to make choices with their traits on what you are going to be supporting. I.E. Say the "Economic Governor" handles resource production. Will you be taking a leader with a mix of traits that cleanly buff trade and alloys, or will you take a leader with traits that increase the job ouptut of all resources from residence at the expense of job output from citizens? There could be a "Law Governor" who is the one that applies your edicts, and they may have traits that change specific edicts, some perhaps cleanly and some perhaps with huge tradeoffs.
Make it so the leader limit is a suggestion. Literally, like the leader limit is exactly enough to staff all positions. Make the only benefit to going over the leader limit is to have a "replacement" on stand-by, in case you see a candidate you like and know that your current leader is getting old. Or for example because you have a very good defensive-war-time economy governor that you want to keep on the payroll in the event that those
democratic crusaders get any bright ideas.
Now, re-evaluating your suggestions without considering the developer's intentions...
* A base Leader level + Leader scaling somehow linked with Empire size
I don't really know what you mean, but if you mean "Leader Cap scaled with Empire size", I think it's a good idea. Here's my suggestion:
BASE Leader cap is enough to fill your council, plus 3 (one for science ship scientist, one for starting governor, leaving one open slot for probably a scientist)
T4 Capital Planets can build a single "leader building" with the following options:
Sector Administration: +1 leader(governor, if segregated leader caps are introduced) cap, governor of this planet will apply planetary traits to all planets (without their own governor) in sector. Limit 1 per sector (duh)
Planetary Specialization Center: +1 leader cap (+1 governor), planetary traits that "match" the planet's designation will have additional effects
Staff Headquarters: +1 leader cap (+1 "free"), planet cannot be assigned a governor (can still benefit from sector governor)
Noble Estates (yes this already exists): +1 Noble Job, +1 Housing, +1 leader cap (+1 governor), governor costs unity to unassign, but if sector capital and non-capital sector planet has this or any upgrade, the planet benefits from both planet governor and sector governor simultaneously.
Illustrious Noble Estates (upgraded from Noble Estates): +1 Noble Job, +2 Noble Servant Jobs, +3 Housing, +2 leader cap (+2 governor), governor costs alot of unity to reassign, but allows a second governor position for this planet. The original governor will apply planetary traits to all planets in sector. Limit 1 per sector.
Orbital Noble Estates (also exists): Same as Noble Estates but with an additional +1 Noble job and +1 Housing, and on an Orbital Ring
Orbital Illustrious Noble Estates: Same as Illustrious Noble Estates, but with an additional +1 Noble Job and +2 Noble Servant Jobs and +3 Housing, and on an Orbital Ring
Eternal Executive Center: For "Permenent Employment". +1 leader cap (+1 "free"). Any leader may be "assigned" to this planet, although non-governors will not apply any bonus and not prevent sector governor's from applying their bonus. Upon death of this planet's assigned leader, that leader is resurrected with a "Zombie" trait.
EDIT: All of these except the Noble Estates and EEC would require to maintain 50 pops (the amount necessary to upgrade the capital building to T4) or they will be disabled. This is to prevent the cheese that works on upgrading the capital building with temporarily re-settled pops. Noble Estates would have the current requirements (a fief needs a lord much earlier than actual administration would demand) but Illustrious Noble Estates still have the T4 requirement. EEC would have the same requirements as "Posthumous Employment Center".
* Significant reductions to the penalty given for going over the Leader cap
Either the penalties are harsh enough to prevent you from going over, or they might as well not exist. There isn't a good in-between, sadly, because different build
currently have different opinions on if the current penalties are easy to ignore or too nasty to dare go over.
* Techs which give increased Leader cap
This is one sollution, though I seriously doubt players will be happy to hear that science will be made even more important. Having a late-game repeatable tech that increases the limit is something I'm surprised they didn't add this patch, honestly. The things that can and cannot be infinitely researched are weird.
That said "Tech" increasing leader limit is weird. An alternative may be a button you can click that costs as much unity as a tradition to increase your leader cap by 1.
* Having a Legendary Leader offer to join you MAYBE giving a free +1 Leader Cap (so you pretty much always get to accept the choice of whether to take them in)
Good suggestion no matter what gets added.
* Make Sector Governors apply their on-planet Traits a bit to the rest of the planets in a particular sector.
The phrase "a bit" scares me a little. Will traits have a "sector" effect and a "planet" effect? But my suggestion on how to handle this is outlined above.
Immediately after a ground combat rework. But there is NOTHING they can do to general to make them relevant at the moment.
Mod that increase the limit to 50. Problem solved.
Why 50? I removed it entirely. Or more precisely, I set it to the stack overflow limit.
In all seriousness, just setting the number to 50 does break a lot of modifiers that currently increase leader cap, which will need alternative modifiers, so it's not
as simple of a quick mod.