There are a few other possibilities. First of all, Montresor himself states that he had "vowed revenge" and that he would "punish with impunity." Revenge, acted out with impunity, means pretty drastic measures. So, we can take it from Montresor's lips himself in the first paragraph that he is going to enact some awful fate upon Fortunado. Granted, he isn't specific there, but, it does hook the readers and get them interested, and we know the entire time what is ultimate purpose is. So, don't skip over that when looking for clues.
Secondly, when he leads Fortunado down to the catacombs, he goes through his own apartment. There are, fortunately for him, no servants in the house that see him. He planned this; he told his servants he was going out for the evening, and to stay in the house. He comments wryly that
"These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned."
Now why would Montresor go to all of the trouble to make sure that his servants weren't in the house? He would do this to make sure that no one could tie him to being with Fortunado, leading him into the basement. This tells us a couple things. One, he doesn't want anyone knowing where Fortunado has gone, when it turns up that he is missing, which means he wants Fortunado to not be found, and two, that he himself can't be implicated just in case Fortunado is found. In a couple days, Fortunado will be reported missing. If the servants say Montresor leading him underground, the authorities would search there, and perhaps find him. This all foreshadows the fact that Montresor is leading him down there, permanently.
I hope that those clues help a bit; good luck!