I've seen that now on the "rosters" tab in my league it is possible to see every team payroll.

Is there a future intention to let leagues, if they want, fix a maximum cap for the teams to abide to? That would be a cool wrinkle for the fantasy experience.

Yeah, 100%. I've been incrementally adding more salary features and there are more to come, including a restrictive cap like you've mentioned here. I don't suspect it will be ready this year but it's part of a larger picture.