1. I'm sure you will be a good author.
2. I'm sure you would be a good author.
3. I'm sure you can be a good author.
4. I'm sure you could be a good author.
(What is the difference among them in meaning?)
will = something that will definitely happen
can = the person has the ability to be a good writer (but nothing is said about whether he/she wants to be!)
would and could = possibilities (nothing said about ability or certainty)