I'm relatively sure you can still be accepted next year, if you're able to pull your marks up. However, the only person who can tell you for sure is an academic advisor. They have drop in counseling in BSB in the associate dean's office, and now is probably much less busy then August/September. If you live in Hamilton or nearby, I would definitely recommend going soon. If not, you can probably still get a response via email, but it might not be as quick.
http://www.science.mcmaster. ca/ass.../advising.html
If you don't get accepted, I know quite a few people who continued to take courses after graduating, and I think you'd be able to take as many as you want. I believe these courses will count towards your GPA, although they might not be counted if you're looking at grad or professional school (so you wouldn't be able to circumvent the required 4 years that way). Again, an academic advisor is the best person to talk to.