After Schengen visitor stay prolongation, what if one flies to a non-citizenship country?

I'm a non-EEA citizen, I don't require a visa to stay in a Schengen country for up to 90 days in a 180-day period. After staying in the Schengen zone for two months, I requested in advance to extend my visitor's stay for another 90 days in the country I've been currently staying. My request was approved, but along with my approval letter I also received a letter which I will need to submit to the local authorities at the airport upon leaving the country. In this letter, the authorities are instructed to promptly send by email to the immigration office the following information:

1. my immigration office case number;
2. the date of my departure;
3. the country I'm departing to.

Now, the most interesting part to me here is the following: when my visitor's stay extension request was being processed, I was requested a couple of additional documents, among which there was a return ticket to the country of my citizenship. I didn't have a ticket at the time, but to satisfy the request I booked a rebookable/refundable ticket to the country of my citizenship.

Now, with that in mind, what if my plans change down the road, and I will need to fly to a different country, not to the country of my citizenship? In other words, will the local immigration office, for whatever reason, find it "suspicious" that, instead of returning to the country of my citizenship, I decided to fly elsewhere?

Personally, I see nothing suspicious, but I think that every small detail may be important in these matters. After all, why does the immigration office want to know where I left, not just that I left in time?

To recap (or ;TLDR):

1. I requested to extend my visitor's stay beyond 90 days, but I didn't specify in my application that I had the intention of returning to my country of citizenship upon the expiry of my extended permit (but, of course, I did specify that I would necessarily leave the Schengen country from which I requested the extension).

2. I was requested a return ticket to the country of my citizenship (I don't know why). I satisfied this request, although I didn't with absolute certainty know that I would necessarily leave the present country of my stay for the country of my citizenship and not for another country.

3. Upon leaving, I will need to submit a letter to the airport authorities, which will promptly notify the immigration office by email when and what country I left for. (Why do they want to know this?).

4. If it turns out I will need to leave the Schengen country of my stay not for the country of my citizenship, but for another country, will the immigration office find it suspicious? What can possibly happen to my future admissibility to the current Schengen country which has approved my stay prolongation, and what could possibly happen to my admissibility to the wide Schengen area?