Boss requirements :
Playtime, and some type requirements are fine actually.
Difficulty :
It's already hard enough for some of them but not impossible. The difficulty on the hard ones is fine since we have to think twice for the team instead of go in randomly.
But as another player said before, it could be nice to have the choice to choose the difficulty level. Higher level for higher rewards. Like the 2 boss during valentine's day event for example. It can be fine for noobs i guess.
Cooldown :
It's not easy to do all the bosses of the game when the cooldown is up. So better not reduce the cooldown if the rewards are reduced too. It takes a lot of time to make them all. And you do that, it will be like mini-boss : not done anymore. In my opinion mini-bosses are waste of time due to their positionning (ho-oh and rayquaza especially). It will be the same for bosses with 5d cooldown.
In my opinion, the bosses are quite balanced for those 3 things. If you plan to do something, i propose to add some new fun bosses like one who required ONLY fire type pokemon or one with full shiny pokemon etc... no need big rewards on them, just for fun.
I guess there are much more others things to remake instead of bosses.