All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
Our assumptions and imaginations have no effect on the destiny that Allah, the Exalted, predetermined for us; that destiny will essentially come to pass in accordance with His wisdom and knowledge. Allah, the Exalted, says (what means): {And ever is the command of Allah a destiny decreed.} [Quran 33:38]
We say: What Allah wills shall come to pass, and what He does not will shall not come to pass. The same applies to answering supplications; it is subject to the will of Allah and has nothing to do with our assumptions.
However, when someone has good expectations of Allah and thinks that He shall grant him what is good, it is hoped that he shall be granted the good that he has expected. On the other hand, when someone has ill expectations of Allah, it is feared that he would incur the evil consequences of his ill expectations of Him. However, this good or evil may not come to pass for a divine wisdom such as testing a person and the like.
The hadeeth Qudsi (Sacred hadeeth) to which you referred was narrated on the authority of Abu Hurayrah, may Allah be pleased with him, from the Prophet, sallallahu ‘alayhi wa sallam, from what he narrated from his Lord, and it was cited by Al-Bukhaari and Muslim. Al-Haafith Ibn Hajar commented on the hadeeth, saying, "It means, 'I (Allah, the Exalted) am Able to do to him what he expects Me to do to him..." [Fat-h Al-Baari]
Hence, the meaning is that Allah, the Exalted, is Able to do what the person expects of Him, and it does not mean that whatever the person expects of Him must come to pass; rather, the matter is subject to the will of Allah, the Exalted.
Allah knows best.