well, i can give you reasons to believe its a dragon thats coming out of that hole.
all you need to know is follow my line and remember the key word: Magic. ( plus understand how dragons work and why they rise. so heres some important articles in the wiki that you should know, even to understand the lore around dragons a bit more:
http://wiki.guildwars2.com/wiki/Magic and
http://wiki.guildwars2.com/wiki/Elder_Dragon )
Scarlet's first mayhem's were just so she could assemble, develop, test, improve, and prepare her forces. not to attack lions arch, but to hold ground to her real deal, a source of magic big enough to mean something. think of someone who just found a huge profit spot and wants to hold and protect that spot because that spot is making him rich.
Scarlet them, starts to look for the said magical source. Magic in tyria flows through everything, everything shares the magic that flow through the world. from stones to elements to plants to living beings, everything in tyria has acess to magic, and because of that magic flows. the bigger the thing or the need for magic ( through manipulation or not ) the more magic will converge to said point, upon getting near and near to those points, the magic start to converge, forming what is a line. these lines are called leylines, now it is where the theory becomes weak, but following my line of thinking, the higher the concentration of magic in one specific area, the greater is the risk of underneath the place there is an elder dragon sleeping.
guess what?
didnt scarlet found one leyline convergence point in the ruins of old lions arch? wasnt her researching dragon energy when the thaumanova reactor blew? and wasnt that exact thumper that went DING? probably meaning that in that place a GIANTIC source of magic was being held, probably, giantic enough to be just like zhaitan's. similar to what happened when the gods come to tyria and settled Arah's heart upon the artesian waters.
also, the drill is turning and grinding right upon where the thumper spot was.
so if you believe in my thinking line, its a matter of " it happened before, its history repeating itself. "