I think there should be additional options to enter leagues but the direct pizza entry fee. My suggestion would be to introduce tickets, especially in regard of the club membership packages you can purchase. I bought the AotA-Expension when it came out. Though the free week of leagues happened to be part of that experience, those who buy the basic edition now won't see much of the new feature, either their pizza runs out on two tries or they spend it otherwise, both leaves a sour taste. Include 3-5 tickets, expressly meant to try out leagues. It would imply a stand-alone option: 5 League tickets, obviously cheaper then 250 Pizza, since its league access only.
By now i came across another interesting implication: Finding tickets in chests, much like you would stumble across legendary treasure. Maybe could even sell them as if they were treasure, in case you prefer some gold and got no interest in leagues.
I really like this idea. I know it's different, but in poker sites (I'm not sure if we are allowed to say the specific sites here) the prizes of the tournaments are not only money, but also tickets to enter another tournaments for free. I think that would work here as well.
This would also work well with free/gold leagues - tickets would make a great prize instead of pizza/figure.