School starts tomorrow so I don’t know whether I’ll have time to flesh this out today, but I want to put this suggestion out there. It actually builds on assorted things proposed by plenty of folks but adds something original, I think. Here’s my speculation on the tomb based on recent things:
1. It is not implausible that it was intended for Alexander and would have been started while he was still alive
2. Of course, Alexander ended up getting buried in Alexandria
3. So Amphipolis ends up with this big tomb and no one to put in it; but putting ANYONE other than the intended occupant in that tomb would be making a political statement
4. The latest news from the site suggests there were great efforts made to seal the tomb in an unprecedented way (I’ll be posting on this later today or tomorrow) … so:
5. Rogueclassicist goes out on a limb to suggest the Amphipolis tomb will turn out to be EMPTY (wall decorations might be there); not looted but intentionally not used.
6. The tomb/mound was transformed into a memorial monument of sorts (everyone knew it was there), with the lion put on top as a sort of generic marker of sorts. The ‘sphinxes’ were beheaded when everything was sealed up because they weren’t guarding anything. Perhaps a symbolic ‘deterrent’ for folks who might have been thinking about using the tomb for themselves.
… I’m hoping I’ll be proven wrong in the next few weeks and we’ll have a magnificent, occupied, Macedonian tomb but this is going to be my working hypothesis for the next few days.
rogueclassicism 
The grave may have been prepared for Alexander: account was held by its size, even if it is strange that we did not want to bury him(it) near the royal family.
Either it is about that of Nearchos, whose political and historic importance does not need to be underestimate. The latter was native of Amphipolis and, died late in life, was immensely rich; furthermore, he embodied after Alexander’s death the authority and the heroic dimension of the first one companions of Alexander and was the legendary character of the epic return by the sea since the estuary of the Hindus. The idea of a grave by default, a symbolic negative of Alexander the Great, is however attractive …
On the other hand, I do not think that sphinges was able to be deliberately beheaded to mean the idea of a grave the symbolic value of which forbade that we can attribute award it to somebody else.
The lion and the sphinges were destroyed and degraded by Lucius Aemilius Paullus Macedonicus and his legions for political motives doubtless bound to the fact that everybody knew that the grave was connected to Alexander or Nearchos and that this place embodied the Macedonian resistance, or could become it.
The grave is maybe empty either containing a very famous Larnax we shall soon know it.
To my opinion is, that the Tomb belongs to:
Antipater ( c. 397 BC – 319 BC)- Macedonian general and a supporter of kings Philip II of Macedon and Alexander the Great.
In 320 BC, he became regent of all of Alexander’s Empire. Antipater was one of the sons of a Macedonian nobleman called Iollas or Iolaus and his family were distant collateral relatives to the Argead dynasty
Antipater had ten children from various unknown wives. His daughters were: Phila, Eurydice of Egypt and Nicaea of Macedon, while his sons were: Iollas, Cassander/dros , Pleistarchus, Phillip, Nicanor, Alexarchus and Triparadeisus.
Antipater was “‘in the shoes” of Αlexander for 20 years ,
«τά κατά Μακεδονίαν τε καί τούς Έλληνας Αντιπάτρω επιτρέψας»,
controlling the rest of Greece , sending troops to Alexander He was the “right hand” of Alexander,
and his son Cassandros was THE ruler of Greece – killing everyone else
( Cassandros , execut Alexander IV, Roxana. – Of more lasting significance was Cassander’s refoundation of Therma into Thessalonica.)
Therefore, Cassander’s fother – Antipater , died in 319 BC, is buried there.
Cassandros was powerful enough to make such a tomb for his father, Antipater.
P.S. :Argead dynasty – (Greek: Ἀργεάδαι) was an ancient Greek royal house, ruling dynasty of Macedonia from about 700 to 310 BC. Their tradition, as described in ancient Greek historiography, traced their origins to Argos, in southern Greece.
It’s hard to imagine that Perdiccas would have built a tomb for Al anywhere but in Aegae. (Side note, the long discussion of the Aegae tombs in Jonathan Hall’s latest book raises the possibility, quite attractive in my view, that Tomb II was originally built for Al and then re-used for Philip III after Al’s body was stolen).
But having said that, I can’t imagine a non-royal like Nearchus having such a grand tomb, and don’t have any better suggestions.
Hmm. I haven’t really been following this, but from the sequence of photos shown earlier this looks like the very epitome of a robbed tomb. The blocking wall has been partly dismantled and the sphinx heads broken off to provide access. The hole in the inner wall provides access to the burial chamber. Presuming they came down (more or less) onto the entrance (rather than clearing it fully as has now happened), the antechamber would gradually fill through the hole that had been created.
Unless the first photo shows the blocking wall partly dismantled by the archaeological team, I fear we are in for a disappointment (and perhaps the mere presence of a hole through the inner wall should prepare us for that, whatever else may have happened).
I like your reasoning, and I look forward to finding out what’s inside. I agree with Le Pendard that the sphinxes were unlikely to have been beheaded for the symbolic reason you suggested (I’d like to see comparanda for that).
Perhaps it’s not a tomb at all, in that no one was ever buried in it. It seems more likely to have been a heroon or similar cult center (probably for Alexander). See http://phdiva.blogspot.com/2014/09/amphipolis-tomb-or-heroon.html or http://amphipolis-tomb.blogspot.com/2014/09/its-a-heroon.html for some arguments why. Under this theory, the monument got back-filled by the Macedonians themselves in an effort to keep the monument from collapsing.