There had been actually Celts in Ancient Egypt. What if somehow Celts emigrate in larger numbers and are a pillar of Egypt's defence, maybe kind of a Pretorian guard?
You're on an odd one spree today - Me Gusta.
A Celtic Pharaoh, are we talking about the Galatians.
Three problems need to be overcome
1) We need to get them to Egypt
2) We need them to be willing to be a Pharaoh.
To be honest, I think this would largely depend on 'Wanking' the Galatians quite a bit - establishing themselves as the sole power in Anatolia. Not easy, and not my area of knowledge to be frank. But to take a stab at it.
Lets have the Celts be dissuaded from invading Gaul - something unites the Gauls into a unified state. Perhaps the fear of the Celts combined with the Romans, and the right leader and the right success brings them into play - yes, I'll handwave that.
A strong Gaul means that the Celts are more likely to move East. This makes the alt-Brennus even stronger, and if allied with the Romans, has the rampage through Illyria and Greece, and onwards - eventually reaching Anatolia again, but this time, much stronger, with more loot, and more settlers. This leads to a stronger Anatolian Greco-Celtic Galatia.
The Galatians ally with the Armenians to roughly split the lands north of the Selucids between them - Galatia to get Anatolia, and Armenia to get the Caucauses and Atropane (if it came to war with the Selucids). That alliance should easily be able to handle the various Greek players and during a Ptolemy-Selucid war that the Selucids lose, take Atropane and 'Lesser Cilicia', threatening the Selcuids.
With a strong, unified Gaul, the Romans are less likely to come knocking about any time soon - giving plenty of time for the Selucids and Ptolemys to beat on each other, allowing the Armenians to expand in Iran and N.Mesopotamia, whilst the Galatians take Antioch and the coast, eventually leading to a border with Egypt.
At which point it comes down to whether this Celtic state can overwhelm Egypt at the right time. Its successfully pushed aside the Selucids with Armenian help - but after that, the Armenians are likely to want to gobble the Selucids alone. However, if the Celts, with their wars, and hopefully a culture tied towards having lots of Celtic babies, could well adapt their techniques, adopt what works from the Selucids, and put forward an army somewhere between the Celtic, Roman, and Alexandrian styles - throw in Elephants that they could adapt from the Selucids - and the Galatians could handily invade Egypt.
So I leave it with a Galatian Empire that is more 'Overlord Tribal Republic' than Empire, or somewhere between the two - but not a Galatian Pharaoh. That is where we have a lovely lovely civil war. In which the biggest guy in Egypt decides that he'd rather have more direct control over Egypt, and not deal with the rest of the Empire - and fights to install himself as Pharaoh in Alexandria. Assuming a victory, we have a Galatian-dominated Levant and Anatolia, and a Helleno-Celtic-Egyptian Pharaoh.
Completely bonkers and handwavium, but I'd love to see the result of a more multi-polar Mediterranean, and having a strong Gaul, Celtia, Rome Carthage and Greece would fulfil that rather than a massive Roman Empire.