There have been a few free maga Gacha's guaranteeing 5* pulls, most people will know the one that required 160 magas as this was considered too high a price for free players but shortly after they got rid of that one the brought out I think it was 3 40 Free Maga pulls that guarenteed a 5* + 9 other units, I remember this as some members unfortunately used their magas during the 160 free maga pull and had none left for these better Gacha's. Some members even got 2 x 5* out of the one Gacha, I was not one of them.
No idea when they will bring them back but always hold onto your magas as they always seem to pop up out of the blue and disappear just as fast.