A friend of mine who is an English teacher told me that the following question tag was correct: 
Ah, what an interesting question, Antonio! Now, usually, the pattern for question tags is positive statement – negative tag; negative statement – positive tag, and here’s an example: It's hot, isn't it?
You don't like hot weather, do you?
OK, so that’s pretty straightforward, but of course, in English there’s always exceptions. And when we use will or shall and we’re making an offer or suggestion, and we want to know how someone feels about it, we can use the pattern positive statement – positive tag, like this:
It's hot. I'll open the window, shall I?
We've run out of time. We'll finish it tomorrow, shall we?
So there’s a couple of examples where you have a positive statement with a positive tag. I’ll open the window, shall I? And it's worth mentioning that in spoken English, it's much more common to hear the short forms I'll and we'll rather than the long forms I will or we will or I shall.
It's also common to find the pattern positive statement and positive tag for suggestions with let's – and the tag again is formed with shall we? like this:
It's really cold outside. Let's stay at home tonight, shall we?
So, when we make offers and suggestions with will or shall or let’s we can use positive statement and positive tag. I hope that’s a useful answer to your question, Antonio!
