You say that the souls of all the martyred saints going right back to Abel are under the Altar. Jesus mentions this in Matt 23:35. This means that seal is open and therefore seals 1-4, as well. Zechariah 1:8-11 and Zech 6:1-8 state those horses were sent out 'to range over the whole earth'.
Therefore the next prophesied event must be the sixth seal, the Day of the Lord's wrath.
From Watchman Bob: Good insight. The context of Mat 23:35 is pointing out those who not only have been martyred in the past but those who will be martyred in the future, at least until the sixth seal is broken announcing the beginning of the Day of the Wrath of the Lord (there will also be those who are martyred after the Day of the Lord begins - cf. Rev 6:11).