The ability to copy the contents of a tweet.
Implemented in v1.6.6
This feature already exists. Long press the tweet in your timeline (or single press) and to the right of the Share sympol is a three dot symbol. Click that, and you get a selection of options:
Block / Unblock user
Report for spam
Chris Atterbury commented
I usually select 'quote tweet' then copy from here.
Johannes Erchen commented
You can share the tweet into your clipboard...