|Saki is out of question. She is pretty much paired up with Wataru now.
Ruka showed much more interest in Hayate when she though he is a girl. Heck she is yet to join the harem.
It was stated that Nagi will grow up into a babe, but the question is how many years it will take her.
Spoiler (mouse over to view)
And it's possible that she and Hayate are related by blood.
Hayate haven't really show any interest in Ayumu. He did however show interest in Athena, Maria and Hingaku.Right now the manga isn't moving in favor of any of those girls (though Hingaku did got lot of "screen time", must've been the rankings).
I wouldn't mind a Hayate x Maria or Hayate x Izumi (don't confuse her with Isumi) ending.