You are in bed with your lover and you hear a car in the driveway. You say "Maybe it is my husband." or "It might be my husband." You do not say "He might be my husband."
Bottom line: "He" is not the subject of this sentence. In fact, there is no word, even implied, meaning "he" in the Czech sentence. (If the word "to" were deleted, then "he" could be the implied subject of the sentence. But that is not the case here. The "to" makes it clear that "it" rather than "he" is the subject.)
It depends on the speaker... I feel a difference here (in this sentence) in that "možná" has a slight subtext of hope and "asi" sounds more like "probably" to me. But people's opinions on the exact meaning of these and similar words vary. "Asi" has a 65-70% likelihood to me, but I found a survey in which people assigned percentage to this kind of words and the average for "asi" happened to be around 55%.
Note that "asi" can also mean "approximately", e.g. "Měří asi 180 cm." (He's about 180 cm tall.)
The word "maybe" means something different from the word "probably". The word "maybe" means "possibly". The word "probably" means not just "possibly" but in fact "likely".
Words have actual meanings, both in English and in Czech. True, in the situation we are given, a speaker might have said "probably" rather than "maybe". Both utterances could be called realistic. But the fact remains that he/she said one, not the other.