Wow... lot's of good ideas - plenty to pick from - but let me add my 2 cents.
First, I think you need to pick a couple of these ideas rather than just a single solution, and I do think some form of "random" events would add a lot to the game. We live in a world of random events, and yes... when it happens to me I get upset, angry, etc. but that IS reality and I would love the game more for it.
On the other hand, I will be first to say that shouldn't be the ONLY adjustment you need here to get your 20/60/20 results (which by the way I feel is a good target). Adding owner characteristics would be nice if you have the coding around it to react to a "bad" owner (results in the owner being forced out?).
Some minor adjustments on how the variables relate to each other... a simple example is that a new stadium will "likely" help attendance for a while even if the team stinks - but on field results must be there or attendance will slip even in a new stadium after a couple years.
I guess my main suggestion. Markus, would be don't make it ONLY a mathematical solution. Realism is randomness. As long as you keep some of that in the game, it will be awesome
Henry
PS: I still remember "Replay Baseball" and its "Rare events" book. I used to LOVE IT when the dice results said "go to Rare Play Book" !!!