I totally agree with what you are saying. I wouldn't want it to be just 3 or 4 teams winning MLS Cup all the time. I like the unpredictability of our playoff system and MLS got it right for the most part, maybe it could use tweaking, but they added the away goals rule which really spices it up. We all know Europe has the away goals rule which is fine, but they let away goals count in (over/extra time) and that's a mistake. MLS got it right by not allowing away goals to count in (over/extra time). The MLS playoff format will be better when the league expands to 24 teams or more.
MLS, while going on 20 yrs, is still in its infancy in the minds of American sports fans. Any attempt to make MLS more like the Euros with 1-3 super clubs is going to alienate a lot of casual fans. Look at the two biggest sports in the USA, the NFL and MLB. I would argue that part of the NFL's popularity is parity. I'm a Bengals fan but I at least have the assurance (recently) that there's a chance my team will make the playoffs. MLB, on the other hand, has super clubs and I'd argue that it makes baseball more boring than it already is.
I think it may be salient to note that the league with no cap (MLB) has produced significantly more unique champions than leagues (NFL,NBA) that have caps. Since 1980, 9 NBA, 16 NFL, and 20 MLB teams have won their respective championships. Though I suspect this has more to do with the nature of the games/leagues and less with the existence/structure of a salary cap.
Here's what should happen. No cap. Instead, everyone takes their actual salary total, and "antes" a like amount into the pot. Cup winner takes it all. That would make you think about it a little bit...
That is effectively what happens now, except there is no ante. What would happen is the rich teams would 'buy pots'. Rich would get richer, as happens now.
Bottom line is if the MLS wants to compete with other leagues for the best talent they have to be able too do so. I like parity but the salary cap needs to go up, until then the league will struggle in CCL and all international competitions.
Personally, I'd rather the league struggle in CCL and other international competitions than we become like La Liga or the Premier League. I'd also rather have true league wide competition than see the best players in the world playing for three or four teams in the league with those same teams always winning everything. The absolute best part of MLS is the parity. No other league in the world has that. Nothing is more important to me personally. So, raise the salary cap, but do it in a way that every team has the same resources available to them to build a team. That's what makes MLS special.
I'm fine with raising the international spots. I don't see where this would have a negative effect on kids growing up and choosing soccer over another sport. Their mindset should see that teams are buying talent, not that they're buying foreigners only. Even with the 8 player limit, you can still have more foreign players on your team if they have their green cards. Raising it would just remove some of the barrier of entry. I'm fine if we're not a feeder for the USMNT. I want to see quality soccer. Having better players around to train with with the US guys can only improve them. I feel that the barrier for most kids right now is cost. We don't have a free academy system where you can spot talent at a very young age and pay for them to train with you. It's a pretty big risk to pay for your kid to be in an academy, to hopefully be good enough to make a good salary, which right now usually isn't in the MLS. I'd like to think that once the salary cap goes up, you start seeing more people choose soccer over other sports, thus seeing the talent pool go up. Over time I hope that acadamies are able to buy/sell youth like other leagues to the point that kids are able to go there for free. It's likely far away, but something that would help bring in better players.
I'd be happy if we effectively emulate the German model: Most USMNT players are in MLS and play a similar system/are familiar with one another; Some of the hyped players go to bigger leagues/teams; Everyone is close to home, so call ups and training are less of an issue; The soccer-loving public knows all the players well.