I was reading all the discussions in excitement for a long time and now, when the time is hopefully close, would like to join in speculations.
Though, unfortunately, I can see more reasons why the tomb can't belong to the candidates on the Nikas' list than I can see why yes. First, my thoughts on Olympia. In the beginning, I thought there was no way why it can be her tomb, the biggest argument being that there was simply not a single person who would want to give her appropriate last honors, even her being Alexander's mother. And then I realized, there was actually one, Thessalonika. Olympia practically raised her and it is possible that Thessalonika loved her as a mother. No proof but it is quite a possibility. So, it maybe the case that T. wanted to give Olympia a burial fitting for Macedonian Queen. As for means, yes, Kassander forced her into marriage but then he named a city after his wife. People do fall in love, even such monsters as Kassander. So, I can envision a scenario when Thessalonika went to Kassander and said, "You know what, darling, the woman raised me, she was like a true mother to me, I want to give her a proper burial. You don't need to publicize it, you don't even need to be involved, but please, do it for me?" And Kassander caved. As romantic as it sounds, it is quite a possible scenario.
Alexander IV and Roxana are another valid possibility (mostly time wise and geographically) but I can't figure out who would want to do that?
I also think that Antipatrides (any combination of them) are good candidates.
And in the end, Alexander is still a possibility. There is nothing to say against the idea that, after learning how Egyptian Pharaohs started to built their pyramids right away after they ascended the throne, Alexander didn't get fond of the idea. Let's not forget, that Dinokrates' plans for transforming mount Atos' geographically very close. The constriction of such a tomb might be not widely publicized and just selected few knew about its existence. Perdiccas would definitely be among those few. Honestly, I was always suspicious about Ptolemy's successful hijacking of Alexander's remains on route to Pella. Even in modern times it is customary to organize a fake transport of some precious goods but the real McCoy transported secretly without much ado. Plus, suspecting that Ptolemy might want to do exactly that, Perdiccas might wanted to have a cause to declare war on his former brother-in-arms. It didn't turn out well for Perdiccas, but he wouldn't be the first (or the last) whose perfect plans went astray.
Question for Agesilaos: you mentioned in one of your comments above that there is a candidate for a burial place for Thessalonike. Can you please let me know where can I read about it? I tried to find, but unsuccessfully.