Mercenaries - you better believe it. In general, troops that were widely used in various armies (Cretan archers, Balearic & Rhodian slingers, as an example) will be available as regional recruitment in very small areas to reflect subjugation (by those factions that would raise native units, given the appropriate government type chosen for that province) and in a wider area as mercenaries to reflect their mercenary activity.

The siege engine - I know there are a ton of guys asking for diversity in siege weaponry. I'm surprised no one has told you what that one is. Well, I'll let you guys guess.