After they manage to defeat in the field 100,000 men field armies backed by a shitload of Vauban style fortifications. Either of the two borders will be a very tough nut to crack and Byzantine will be getting an even tougher target once the full switch to fiscal-military state is complete and the imperial armies hit the 300-400,000 men mark as a result. And the future of the Ottomans lies on whether they will be able to match the economic evolution much more than the military-technical evolution where Iskandar's reforms apparently produced European style infantry.
If I had the choice of Vauban Antioch vs Vauban Mesopotamia, I'd choose Vauban Mesopotamia.
I'd love to see that fiscal-military transition completed, and used ON the Ottomans to produce the border, and that being the impetus for an Iranian counterpart. (Ideally though an Ottoman collapse and a new Iranian Empire emerging from Madarasan IMHO).
Gah, I need some epic Roman successes. I'm feeling exhausted from Roman almosts and not-quites.