Argthtjtr

I want to say if she decides to have a daughter one day.

"If she has a daughter.." <- if she has a daughter right now

"If she would have a daughter.." <- sounds almost right. But sounds like a choice. not statement... If he will clean the room vs if he cleans the room.

"If she would have had a daughter.." <-this means in past if she had decided to have a daughter.

So

if she haves a daughter sounds most right. :D

You'd use the future periphrastic mood:

"If she were to have a daughter one day..."

The future periphrastic mood can also be expressed as follows:

"If she were going to have a daughter one day..."

I list this one second because I like it less. First, I'm a huge fan of being succinct. Second, I feel like it has a different tenor, almost like it places more doubt than the first, but maybe that's just in my head.

Anyway, the future periphrastic mood employs the past subjunctive but then conveys casting the action into the future by way of then employing the infinitive mood in accordance with its futurate sense.

All of that said, you can say:

"If she has a daughter one day, she would..."

The above uses the present subjunctive of "to have" to convey a future hypothetical.

In conjugation, the present subjunctive mood is identical to the present tense, so it's hard to see. The only verb that's different is "to be" (e.g., "If she be a mother one day, she would...), but in the vernacular, English speakers overwhelmingly conform it to the present tense conjugations, much like English speakers do for the future subjunctive but to an even greater degree.

Anyway, what unmistakably takes the action out of the present and casts it into the future is saying "one day" followed by employing the future conditional mood.

Another way you could say it would be:

"If she has a daughter one day, she will/is going to..."

Now this does use the present tense, but it still casts the action of the first clause into the future by saying "one day" and then likewise does so in the ensuing clause by using the future indicative.

These perfectly acceptable ways of conveying future conditions notwithstanding, if your intention is to most clearly show from the start the hypothetical as being futurate, use the future periphrastic mood as outlined above.

By the way, the use of "past" and "present" to describe subjunctive moods in English is a bit of a misnomer since we don't really anchor them to only that time. It's been suggested that's evolved because they form an irrealis, an action that isn't real and thus hasn't happened, isn't happening, and may not ever happen. Whatever the reason, know that "past subjunctive" is called "past" because it mirrors that past tense in conjugation, not because it's limited to being in the past, and present subjunctive uses "present" because it mirrors the present tense in conjugation, not because it's limited to being in the present. Now, the "pluperfect subjunctive," which is called that because it mirrors the pluperfect tense, actually is limited to being used in the past, so it's the exception to the rule.