As neat as it would be Riot won't do it because they won't want to balance around it, which is the only real reason most of the game modes we get now and then aren't permanent, like URF that people scream for, because had it stayed for another week or so half the champions would have been banned.
As fun as it is it just won't happen, far too much work for Riot (just being honest) when there isn't much reward for doing so. A new map is a big deal and the lap actual new map they did was dominion and...well no one plays that. Not trying to be negative just stating it from their perspective.
No Howling abyss wasn't new, they reskinned the tutorial map
No twisted treeline wasn't a new map, it was a reskin of the old 3 v 3 map.