1. First of all, bond between Gray and Juvia must be broken or at least changed. Juvia in 100yq is a parasyte (Relax! This is how I see it. This is not offense)
2. Gray is the best (in my opinion) FT character and obviously one of the best FT characters. He has wonderful background and storytelling. He is a type of character who moves the plot. But he don't get enough time for it. So give my boy the time.
3. Gray needs a big power up. Very big to be Natsu's real rival. May be possible alchemy helps with that.
4. Give him strong enemy. Of course not dragon god but at the same time strong.
5. Stop telling he is weaker than other characters. Especially in form which Cana said. Especially telling he is weaker than Mirajane because Gray with DeS magic must destroy her easy.
For some reasons Mashima does't like Gray who is the most interesting member of team Natsu. He is side character now.