Different raid fight time based on boss level and number of players

I'm reading posts a lot since the gym update and some complainings about the raids seen to repeat.
- Gyms with few players when the raid starts and/or rural players are suffering because their numbers isn't enough to stand against almightys level 4 bosses
- In other hand, players in great areas are starting to find raids boring and unsatisfying for being to easy and now are collecting a lot or Tyranitttars, Lapras and Snorlaxs.

What I suggest is that the 180 seconds battle time changes based on the Boss Level and the numbers of players! The battle times would last between 60 or 300 seconds.
I think that will be a nice way to make raids more fair and fun.
Three or four players when fighting a Tyranitar would have 5 minutes do defeat it. But twenty players will have do to the same in only 2 minutes.
Kill a 18k Machamp with 15 lvl 30 people in 180 seconds is a pice of cake. But to do it in 60 will be very exciting!
The strongers players or teams with 2 or 3 will have more time and face a challenge.
That would also prevent the all "enter a public fight and hope for spoofers to appears" because with increased time people will try to fight solo or in public small groups.

What you guys think?
[Sorry for the rattatastic english; I did use the search, if this suggestion was already made, I apologize]