Came here to follow up on the same issue.
I think allowing teleport only on towers and wards is the way to go here.
Towers are generally considered a safe haven and exert a lot of indirect map pressure by being left alive or taken down respectively.
So it seems only fair to make these viable teleport targets to put emphasis on their overall role on the map.
If they were the only target to tp to, however, tp would be essentially useless in lategame, when all but the t3 turrets are gone.
Which brings me to the point of wards:
Riot already stated in their preseason gameplay preview that the ward meta is gonna get hit again as to receive a shift of direction even more towards being a game that the team has to play as a whole.
So if wards are the only available tp target in later stages of the game, teams get another incentive to up their vision game in terms of providing wards in key tp positions and on the other side deny those very wards to their opponents.
This was my take on the teleport summoner spell and now /discuss
-Insert random statement about english not being my native tongue and excuses for possible mistakes in the post here-