well that's mainly just naming, you don't have to call it that, it's same thing anyway
transfer windows exist. especially that the leagues are seasonal and you have a longer break in summer and winter. generally best part of fm was replacing old guys and hiring new teens in between the breaks. and it's a serious issue if you can just poach your own league players all year around, that's just ruins the balance further, and then you win more and ruin the balance even more which escalates more.
you can call it the long winter or the summer peace time, same deal.
salary caps also make sense. right now you just make too much and limited too much. so it's easy to concentrate all your earnings into a few superstars and then you do well and that also escalates.
the main difference from a sports game it's that people can die. and years are faster but some races live longer so you can adapt to the retirements by making good choices.
you are kind of wrong on the realism. it's an estabilished word. the league is estabilished. 90% of teams usually belong where they are. 10% is some teams that had a bad streak or disasters, even they are only good enough to survive in the next league. it's very rare that teams just advance 2 leagues up. so it doesn't answer the logical questions and doesn't explain the economy. if it would be that easy other teams would do the same. and if you are not the best team in the region you can't poach the best adventurers.
tavern gen should be way less sure, you could know a range of skills and values and risk it if you want but you need a lot of downside, like new people should have bad fitness and need a while to adapt. if your backwater team is 0-40 and everyoen in your league is 0-40 then the guys on 35-40 are jus t the same superstars as a 90 guy in top league. each league should have different values, issues and stuff. sure, the low league issues should be easily fixed by a bit of sparing and thinking and management. in fm that was improving youth facilities and the stadium. so each year you get better kids and sell them on better prices and invest more easier when you do so. backwater should be bad teams because thye are mismanaged and greedy/wasteful to basic rules. for example in backwater insults result in fights, deaths, players leaving your team etc. in higher leagues you could have oiptions to help/manage/ignore those. that would give a variety in the world and make the leagues realistic.
it's just a difference in scale. if you never saw a superstar, you don't have a way to compare them to your guys and relatively you can be very strong and outdo expectations and requirements. fame in fm was big, you didn't know as much about them and you didn't get good offers for them. you would need to capitalize in form and temp results and swindle the other teams with the momentum. randomness, scout bias and skill levels could help on that. limitations too. you can invent a lore for that. like a worldwide adventurers guild pays every team the same, maybe more bonuses for the big ones, but small teams could do a lot of small quests and recieve a different currency which is better in some cases.
I would imagine a small league with a ton of deaths but a few breaktroughs and then reinvest that money into more and more members.
loyalty to a region is one thing but still realistically people would visit all teams in a region, so if there are like 80 in 7 regions then each has like 10. I havent went deep into the lore. so they would still go to the big ones. there might be reasons to go to yours. that would be not sports like. for example you could have a castle/home/dwelling/cave system, so each team could give a room/house and work position for like 3-4 people. so they could settle at your place which is a huge cost reduction for them, this would also naturally provide balance, as each team would have a heavily discounted star roster. that could be linked to research/buildings, like 2-8 of those rooms with buildings, then you could save up and have up to 8 high maitenance players in your squad, long term you would always advance to the next league. maybe 9-10 would be exponential cost.
wrexham also spend double of others. players aren't there for their good of their heart. those teams have crazy recompensations if the players are fired, high bonuses, but also high penalties for not meeting results. same reason red bull teams started in 5th german league and went to 2nd league, money. but you can say the reverse applies for teams like leicester who lost a ton of players with relegation. those are special contracts for special circumstances. in fm you could do similar, give lower salary than required but huge bonuses for goals or wins, so you saved money if they perform bad. ofc if players have different personalities and requirements and you can't finalize tavern recruits easily that also saves a bit.
also, it's nto a fun game if it's easy to have an average squad value of 90+. people want good results and easy game, but good game balance is hard and rewarding. good choices are rewarded, bad choices are punished. attention and time is rewarded.
it would be still fun to have 1-2 league advancements with just veterans that soon retire and very young local guys with no info about them.
randomness is fine. but then it should throw a wrench into your plans.
I'm thinking the nemesis system of middle earth games. like some teams would scale with yours and give you trouble each time. and if the balance is broken, the world changes a bit. for example if some vets retire or die, you get higher stat recruits or higher potential guys. if people train up, the risk of death would rise worldwide until balance is restored. and this could be in the background and have some randomness or delay. then it would be hard to anticipate that.
another feature was nice in mount and blade. when one kingdom was attacked and near the end of losing the last cities, they started hiring more people, fight dirty and rail up harder. so it was very hard to take out a kingdom, you had to do it in a one fell swoop. that also applies to ck3, some underdog saving mechanisms. the backwater could have some of that. like you could send assasins to ruin other teams and it would be unlikely to be found out.
so by balance I don't mean reduce the swings. I mean make it overall balanced and perpetual, with choices having upsides downsides and a lot of risk. some clues to what is the good solution. and a lot of minigames and systems that are different at each league.