Hmm, you've got another 4 months left this year...how many of those characters will be introduced? You could always wait a month or two, poll your audience to see what they'd think, or produce one of those 16-month calendars (so you could still sell it well into '08).
I think that with your art, you could probably do it, even this early. It never hurts to start thinking about merchandise early so you can be prepared when you really need it. Even if you didn't go forward with the calendar, if you had a bunch of prints already made, you could save them for later efforts or sell prints (either online or at conventions.)
No matter what, I think a calendar with spoilers would be popular
You could, also, sell a digital calendar. My brother is a member of 3D desktops UK (
http://www.3ddesktops.co.uk/) who sell a monthly wallpaper calendar. It's just a nice image with a small calendar in the corner. You could sell one monthly for a couple bucks if you didn't want to make a print calendar. That way, you could even tailor the images to what would most "tease" all your readers.
Something to think about maybe?