Which is more correct to use: "May Allah open up for you" (فتح الله عليك) or "May Allah open up to you" (فتح الله لك), taking into consideration that "We have opened up for you" (فتحنا لك) appears in Surat Al-Fath and "We have opened up for them" (فتحنا عليهم) appears in the context of punishment?

There is no harm in using "May Allah open up for you" (fataḥa Allāhu laka) or "May Allah open up to you" (fataḥa Allāhu ʿalayka), as both are correct. The transitive use of "fataḥa" (to open) with the preposition "ʿalā" (upon/to) is used for good things and has been mentioned in the Prophetic Sunnah, such as his (peace and blessings of Allah be upon him) saying: "When Allah opened up Mecca for His Messenger (peace and blessings of Allah be upon him)" and his saying: "until Allah opened up to them."