Rather than a slow visual fade, I'd prefer to have a slightly longer audible warning -- one obvious possibility is a ticking clock getting gradually louder for a minute or two; another choice might be a recording of a train arriving, from the rails singing to the doors opening :-) -- and synced up with a short visual fade so they "arrive" at the same time.
I find the long visual fade is like a child tugging at your sleeve.
Re: Feature request
You'll be able to do that in Time Out version 2. It will support multiple notifiers, and you'll be able to schedule them more flexibly, e.g. to have a two minute sound play two minutes before the break is due.
Those do sound like fun sound ideas, too!