Well as I said they are possibilities but it took Luffy two years to get where he is now with his Haki is there enough time for Oda to fit that progression in and even if he did a full body red hawk wouldn't be able to keep it stabilized and would only be able to keep that form for a few seconds due to getting burned.1) Haki- he can improve Haki alot, make it like Vergo(full body), then maybe use "full body red hawk" or something(so he's all in flames)
2) Gear 4th where can use both 3rd and 2nd together.
I think if he gets any of those he would be much stronger.
Also I can't be certain but is that what 4th gear is? combining both 2nd and 3rd...