I remember getting REALLY pissed the year they offered extra Teambuilder slots as a Preorder bonus and even sold unlimited slots as an add on and then still limited us to 12 teams in a dynasty. That really bothered me.
The #12 has been going on since last gen. I believe on last gen they used to max out the dynasty users at 12. I know it's always been 12 on this gen but it's like it was something they were limited to (probably for legit reasons at the time) back on the old systems and just decided to keep it the same on this generation of systems.
But I hope you're right about that number going away on next gen. I'm interested to see if they implement Teambuilder on next gen or if they just get rid of it.