Bots should be able to temporarily/permanently block users. When a user is blocked by the bot, the bot shouldn't receive any updates from the user.
Reasoning: Users can send messages on telegram very quickly. A bot, even if it has antiflood code, may start lagging for everyone, just because a few users flood messages or inline button clicks and the bot can't cope with the updates anymore: It can't handle the updates as quickly as they are incoming and all update processing is delayed.
This would also stop the need for developers to implement an antiflood / user ban system to every single bot. Currently, my bots handle it this way: If the bot notices it gets spammed, it temporarily "bans" the user by setting a database record. On every incoming message, it needs to check (with cache) whether the user that this message is from is currently banned, and if so, ignore the user. When a message flood comes in, that just doesn't work out anymore.
If bots could block users, bot creators could focus more on the core functionality of their bots and not on preventing their bots from being brought down by spammers.