I really like basketball as a sport for it's dynamic back and forth play and how quickly plays happen. In basketball the offensive "push" of a team is limited to a certain amount of time.
At this point in time in 5cp tf2, there's a gentleman's agreement that the team that controls more points is held to make a play if numbers are equal (since the roundtimer running out potentially will put them in a worse spot than before).
I think there should be an incentive for the team with more points to push, beyond the current gentleman's agreement.
Example: If a team caps a point they have a certain amount of time to push the next one (for example 2-3 minutes), after that time the team on the defense gets an advantage (for example a global banner effect). Thereby teams that have a lead and prefer to trickle down the timer even though it's theoretically their turn to push will be punished.
I'm not trying to belittle a playstyle that controls the game and relies on making your enemies push and making mistakes, especially at the higher levels. I'll even give credit to the teams that have the discipline to pull it off. The only problem with it is that, how Gentleman John has mentioned before, unlocks and certain classes are stronger in a defensive postion, thereby making this playstyle the strongest one and others unfeasable. And i don't expect any reasonable weapon balancing from valve to change this anytime soon.
My proposal would turn the gentleman's agreement into a rule of sorts and punish teams for parking the bus. Obviously there are flawless where it works counterintuitivel (for example leapfrogging on mid, where the team losing the point would already have an advantage to retake, on the other hand, if the timer is long enough and they wait for it, they'll lose their original (uber-)advantage.
Sorry for the wall of text!