Can we just have a giant circular map with no jungle, no turrets, no minions and just have an all out brawl? Start the game off like in aran where everyone is level 3 and has 3 skill points. You can only buy when you die and stuff. The winning team would be the team that gets the most points by 30 minutes?
You wanna have a fight?
this would be a good way to practice mechanics
Could even just have regular rift, however the win condition changes from "kill the nexus" to "score 50 points" or as you said, 'get the most by 30 minutes'.
I would play it then, because it would still retain the same balance, however, players are more aggressive. I dont think the meta would be completely changed, though players would look at 
and think "lots of aoe damage, stasis/heal ulti, lots of cc for team fights" and leave out the "lots of waveclear" part. This is a single, and rough example, of course.
I was also think perhaps leaving base towers completely invincible, however, keeping all other towers vulnerable as an objective of gold value.
oh man, what if the game ended when a team has slain dragon 5 times... or best of 3 baron fights...
Something like Bloodline Champions? Could be fun as a featured game mode.
A Deathmatch?
Yeah we have that, it's called Dominion or you can just dive into the Normal Blinds.
or you can try a free for all map. http://boards.na.leagueoflegends.com/en/c/summoners-rift/IEGvJisL-free-for-all-map-anyone like that.
