If you apply in June of Junior year and don't get admitted, you may choose to apply the following year.
The application will include your senior year grades which may increase your gpa and bump your application. You may choose to retake the MCAT in Spring of Senior yr and have improved scores. You may have done reserach, completed a thesis, published a paper, presented a poster, given a talk, done some community service, served as a class leader, been the graduation speaker, or done something worthwhile during school breaks during senior year. You may have continued volunteering or getting other clinical experience during your senior year. Activies during the summer after junior year might also be on your application.
Depending on how late you want to wait to submit the AMCAS and how lucky you are in getting hired for a job, you might have full time employment to list on your AMCAS as of the summer after graduation.
Unless you've wasted the last year, the application should be better a year later.