It would've been great if you'd expanded your question.
This implies that the there is no longer a possibility of the question being expanded by whoever "you" is, i.e. it was deleted or locked, or the speaker/writer believes the asker is not returning.
It would be great if you expanded your question.
This sounds good, and this construct is frequently used (esp. in a business setting in my opinion) to politely ask someone to do something.
It will be great if you expand your question.
This means the same as the sentence above. I agree that it can come off as too "mandative", as you put it, if the listener/reader isn't aware of your tone, so I suggest the second sentence.
Can #2 be taken to mean "It would be great if you expaned your question (in the near future)", or would it always mean an unreal/improbable situation in the present,
"It would be great if you X" will usually be interpreted as a request. "It would be great if {third-party} X" can be interpreted as a indirect request if the listener has the power to influence the third party.
Something like It would be great if a new road were built here has a conditional meaning - there is not actually a new road, it's a condition to "it would be great." It's not always used to refer to improbable events (is it really impossible that there will never be a road?), though it can - I would freak out if I ever were to see a unicorn in real life.
I was hoping ~
When stated after the event has occurred/not occured, it implies something (slightly) expected and pleasent came true/ the opposite came true. The expectation doesn't have to be something you were 100% certain of, it could be more like 60% (I think it is possible, but I dont know for sure). Tone of voice and emphasis will tell you which way:
I was hoping he'd pick me... (he picked another person)
I was hoping you'd buy a large pizza... (they bought a small or medium)
A dissapointed voice means that the hope didnt happen. If any words are emphasized, they are the relevant part of the wish.
I was hoping you say that!
I was hoping he'd be here.
If the tone is happy, it means the wish came true.
Note that in the above, the statement was made after the event happened or the opposite happened. If stated before the event, then the speaker is expressing their desires. It can sometimes also be a request, but it feels a little demanding.
I was hoping you would take a look at my computer
A more polite version of all three of these is the "could" version.
Because "could" is a form of "can", it feels like the situation is responsible, not the people.
I was hoping he could make it to the party... (but he did not make it, maybe he was busy with work)
Just as before, the speaker is expressing dissapointment, but is explicitly not blaming anyone.
I was hoping you could eat with us (as they are all at dinner together)
This expresses joy, but makes it seem like they were doing you a favour. It's an indirect way to say thank you.
I was hoping you could help me with my homework
This is a polite request.
Anyways, for the non-requests versions, you can change the tense from simple present to present perfect (notice -> have noticed) and it means basically the same thing. But it adds the nuance that the wish coming true or not has some significance to the present.
I was hoping you would have been home (but since you are not, you will miss Game of Thrones)
I was hoping you'd have known the answer (and because you did, you were able to tell the answer to me, yay!)
Best Answer
This is the most grammatically correct usage. You should use this.
This, while technically incorrect, is still used a lot by native speakers.
Your other two sentences are in a similar situation:
This is correct. While normally we use was in the singular case, for hypotheticals we use were.
This, while technically wrong, is still used all the time by native speakers who don't understand the nuance of the subjunctive mood.