I am sure if they do a flash-forward, and show Nagi all grown up with a more mature personality, a HayateXNagi ending will be more widely accepted by fans.
Also, I have no doubt that a 15-17 year old Nagi will be on par with Hina, Maria and A-Tan in terms of cuteness/sexyness
They could always pull a filler chapter where they use some device to show what Nagi will look like 3-4 years in the future (i.e. like Detective Conan) to plant seeds with fans.