graduate admissions - Can I apply again in March for the same intake after being rejected in December? This is a hypothetical question pertaining to the entry of Fall'19. Suppose I apply in December and get rejected. By next semester, I predict my grades would have improved a lot, crossing me over past the 7pt grade line out of 10, which a substantial thing. Additionally, by January I will be involved in a research project which will definitely give me brownie points. Given that I think my application's strength will improve, would it be allowed to apply again? Will my application be considered afresh in March?

Or, should I just wait till March and apply? I hear that intake chances are higher in December, since there are more seats to be filled.
You are trying for an equation containing two incommensurate things. It is an impossible task without a lot of statistical research. You will just need to make a decision. Check the university's rules, however, to see if rejection comes with a prohibition to re-apply without at least a substantial wait. Such rules should be public knowledge and easy to obtain from admissions officials or web-sites.