Here's what I look for in a theme league:
1. Not too limiting on players - I want to be able to use players that *I* like. If I don't have a chance at using some of my favorite players, I'm not interested.
2. Balanced & Fair - Owners who join the league late (or have to draft near the bottom) should have just as good a chance at winning it all as the guys that get in early. The EOL is a great example of this, and one of the reasons why is...
3. Appropriate Salary Cap for the Purpose of the League - Basically, the higher the cap, the more unbalanced a league can become when it has a few (or many) restrictions based upon it. The EOL's cap right now is $50m. This is perfect. It allows franchises like Denver and Charlotte to compete with goliaths like the Lakers and the 76ers. If the cap was higher, there'd be no stopping those latter squads. As it is, *any* franchise can walk away with the EOL title. Also, the cap needs to be high enough that it doesn't eliminate the first thing I listed.
4. Diversity of Players - I love themes where everybody has (mostly) different players. For this reason, draft leagues are absolutely my favorite. They are a challenge to run and probably the biggest challenge to play, and as time has shown, the sim really will only support a couple of them going at once. But you don't have to have a draft league to have a diverse player database. A good example of this is with Gerry's new(er) leagues that put a maximum number of rebounds drafted for a team. Both leagues I've played in have been very diverse and it was an absolute ball to make both teams.
But picking a random salary cap number and saying "no clones, rookies or waivers" doesn't encourage diversity. The top owners will likely see how many top tier players they can squeeze together and you end up with 4 or 5 teams at the top of the league that look the same. For example, there once was a $57m cap league with those rules. The team at the top of each division had this combination of players: '67 Wilt, '92 Worm, '90 Jordan, '09 Lebron. Three of the teams also started '09 Troy Murphy (used to be a beast with 96 defense and 100% SF effectiveness). The fourth team had '03 Ben Wallace and used 09-10 Lebron instead of 08-09. The finals was a matchup between the exact same two starting fives. How... meh. Things haven't changed much really, so the higher the cap, and the fewer the restrictions, then the more identical the top teams will end up being.
With all that said... I've been working on building a team for your $50m open. I think that the number is slightly too high to encourage variety, but it's close. I think $47m or $48m would be perfect for an OL without rookies. I'm not suggesting that you change the $50m; I'm just stating my opinion.
5. Competition & Rivalries - If I look at an advertised theme league and see other owners that I respect expressing interest, I'm much more likely to join. Several really good theme leagues have come to fruition because a newer owner had an idea that excited a veteran owner, and when that vet started giving advice, the originator started working with him to craft something great. When an owner like myself sees this happening, I'm much more likely to join that league. By contrast, if you immediately show that you are unwilling to craft what could be a better league, get defensive or just plain rude, it will often drive vet owners away. I've seen it happen several times: a newbie owner has a theme idea, posts it in the classifieds, gets some feedback from a vet, another vet pipes in and agrees, then the originator gets angry and rude... and guess what? Those leagues die. Every time. There aren't enough theme regulars to come in, be an *** right away, not listen to what more experienced players have to say, and expect people to play with you or join your leagues. It's a matter of respect.
Basically everything comes back to the salary cap. Determining the proper cap to match up with the particular restrictions that you want to run is very difficult but is, in my opinion, the most important part of coming up with a good theme.