Change Legendary chance to pseudorandom

[Cut-Me-Own-Throat-Dibbler]

So would it be based on

-Ative game time
-Waves defended 
-Bots killed
-Ative wins
-Solo points?
...

or a mix of all or some of them.

Could be anything, really. Either a complex calculation like the one you mentioned, or a simple counter for how many boss waves a player participated in. Whatever the mechanics would be, though, I'd keep a lid on the details in order to prevent people from cheating the system.

However, as long as there is no indication that the current system of total randomness is ever going to be changed, discussing the specifics of such an idea is quite moot anyway.