It would be very nice, when typing "/" anywhere on the interface (without another text field focused, anyway), for Roll20 to focus the chat window and enter the "/" to start a new command.
Likewise, the double-quote key would be a good hotkey for focusing the chat window without entering a leading "/", so you can type a regular message.
Both of these are standard in the MMO world. A useful addition would be the ":" to start an emote, which comes from the MUD world, and is sometimes implemented in MMOs.
Are there any other obvious candidates for global hotkeys