So in the preview for tonight's episode of Arrow, the second episode of the three episode season finale, entitled "Streets of Fire", briefly shows Malcolm Merlyn in his Dark Archer's suit (though without the hood over his head), saying "the city has fallen". It doesn't show who Merlyn is talking to, but my guess is either Thea or Oliver. If he's talking to Thea, my guess is that he is rescuing her from Blood's men (in last week's episode one of Blood's men is taking over the same train station that Thea is at) and taking her out of the city and may only be playing a small role in the remaining two episodes. But if he's talking to Oliver, my guess is that he is allying with Oliver and that they will team up to fight against Slade and Blood. By himself, Oliver has a difficult time fighting even one person on Mirakuru, now Blood has a whole army of them. Merlyn might be a better fighter than Oliver, though Oliver is younger and faster. Oliver also can't seem to be able to do any good when he fights Slade, though he apparently beat him on the island (shooting Slade's eye out with an arrow and thinking he killed him), but Slade's return seems to be messing with Ollie's head and he is losing his confidence (that seems to be fairly common with him, he loses he confidence against certain villains, then he finally is able to get his confidence back up and eventually prevail). Merlyn on the other hand seems to be super-confident (like how he reacted when Moira told him about Ra's al Ghul).
So what do you guys think may happen. Merlyn has motive to team up with Arrow, in order to help protect Thea and avenge Moira. Perhaps he'll use one of his earthquake machines to help take out Blood's men in their underground lair, though they look like they're above ground now. Or maybe he'll die as a hero, rather than dying as a villain, but I don't think that's likely (I think the writers probably want to keep the character around for the next season). While he may help out, I think it will be up to the Arrow to actually defeat Blood and Slade though.