Several current theories.
1. Saeki's betrayal is a deception meant to trick any spies and his army will be a Trojan Horse into Osaka Castle.
2. Saeki's betrayal is real, but Toranaga knew about it and has some secret plan he will enact.
3. Toranaga will somehow utilize Blackthorne and his ship to turn the tide of battle.
4. Toranaga will enlist the Portuguese and possibly win the Christian Daimyos/Regents to his side.
I have an alternate theory based on what happened in real life history.
As we all know, Yoshi Toranaga is based on the real life Tokugawa Ieyasu, the founder of the Tokugawa Shogunate, which lasted from the 1600's to the modern day Meiji Restoration. Ieyasu won all of Japan in the pivotal Battle of Sekigahara.
Ieyasu's Eastern Army of 75,000 men defeated the numerically superior Western Army of 120,000 men led by Ishida Misunari (played by Ishido.) The huge part of the reason Ieyasu won is because he convinced one daimyo and one commander of the Mori clan on the Western side to switch sides during the actual battle.
Kikkawa Hiroie was a commander of the Mori clan who were waiting on a mountain to attack on the flanks. During the actual battle, what he did was refuse to comply and attack - stating he was busy eating and asked to be left alone. This in turn prevented the Chōsokabe army, which deployed behind the Mōri clan, from attacking. One commander, secretly allied with Ieyasu, essentially prevented a large contingent of the Western side from attacking through inaction.
Another daimyo who commanded around 16,000-20,000 men, Kobayakawa Hideaki, actually became a turncoat and attacked his own Western army. He had an agreement with Ieyasu to switch sides, but was initially very indecisive during the actual battle and refused to do anything. Despite messages to attack from his Western Army allies, he was reportedly so unsure what he was going to do that he didn't do anything. His army was on standby.
Then Ieyasu forced Hideaki to actually make a decision by ordering his army to shoot arquebuses at his army. This made the indecisive Hideaki to actually choose a side. He ultimately attacked his own Western army's flank. This in large part led to the defeat of the Western Army.
I believe Saeki will follow a similar path. I think Toranaga will get Blackthorne to escape on his ship and reach Edo. There are some troops in Edo (remember when Buntaro offered to esacpe and gather reinforcements from Edo.) Blackthorne will relay Toranaga's order for the Edo troops to start marching to Osaka Castle.
Then at the gates of the Castle at the the battle, Saeki will be indecisive and then choose Toranaga's side in a pivotal moment. This was foreshadowed in Episode 7 when Saeki expressed regret at having to go against family.
Video about the real Battle of Sekigahara.
