My Idea is for a "Stop button" be added to the campaign schedule page.
At some point using Marketo you may run a campaign which involves an email which you then realise you have forgotten something. This could be as small as not including yourself in the smart list or major like an error in the email subject line.
If you run a campaign and realise after you have clicked run, the only choice is to delete the campaign and start over again. A stop button would help avoid having to recreate complicated campaigns.