My suggestion of the soft cap with luxury tax:
Salary cap at $2.5 mil
Minimum salary cap (75%) at $1.88 mil
Luxury tax ($1 for $1 over) starting at $3 mil
Each team can have at most 1 DP (can't be traded). A DP is defined as a player who is exempted from the payroll but cost $1 mil in luxury tax. Teams who are at or below the salary cap ($2.5 mil) will receive luxury tax revenue.
Why would this form of salary cap be superior to the current strict hard cap + DP (that cost $425,000 against the cap)? The answer is growth. All teams are not created equal. There are some teams that have much higher growth potential than others. However, because of the strict hardcap/level playing field, teams that have strong fan base and very high grow potential are not allowed to grow. Under a soft cap with luxury tax structure described above, a few MLS teams like NY, LA, Tor, Sea, Vancouver could have better players. As a result, they retain their current fan base and attract new ones. Potential attendance: Seattle (35,000) Toronto (30,000 in expanded grass BMO) Galaxy (27,000) NY (25,000) would push MLS forward to new height. Which could mean better media coverage, better TV rating. MLS image and reputation among existing soccer fans would increase from better result in the Champions League. Right now, the perception of MLS as a poor quality league is common among many existing soccer fans because of MLS's poor showing in the Champions League. Got beat by 6-1 on aggregate by a team from Trinidad and Tobago and 3-1 on aggregate by a team from Panama only proved their long-held belief.
Another reason why a NBA-style salary cap would benefit MLS overall is that it would give the expansion teams a head-up. They could come out the gate and do well provided that spend and pay luxury tax. The expansion teams have far greater potential than most older MLS cities because they have not gone through "brand damage" that most MLS teams have had. Having a level playing field/hard cap would hinder these expansion teams which have the most potential for growth. The very teams that MLS will depend on for further growth.
Season Tickets:
1996-41,995
1997-37,893
1998-38,683
1999-43,782 (league average was 3,649)
2005-48,300 (league average was 4,025)
2007: -------------5226 average
New expansion teams:
Toronto
2007~16,600 (with 15,000 waiting list)
2008~16,600 (with 15,000 waiting list)
2009~16,600 (with 15,000 waiting list)
Seattle
2009-~25,000
Philly 2010
2010--expecting about 12-14k (currently at 7,000 with 9 months to go)
Vancouver 2011 sold 5000 season ticket deposits within 48 hours.
Portland 2011 sold 4500 season ticket deposits within 3 days.