There really wasn't any significant uprising against the Tokugawa during sakoku - the last real rebellion was in Shimabara just as the policies were coming into effect. So there really is no obvious point for an early overthrow of the Tokugawa. Potentially destabilizing forces did exist (the tozama lords, famine, plague, earthquakes, tsunamis, volcanic eruptions, squabbling Buddhist temples, the rival imperial court in Kyoto, corruption and in-fighting in the Edo court, a rising merchant class, increasing government censorship, hidden Christians, potential succession crises at the start and end of Tsunayoshi's reign, foreigners, etc.) that could work together to overthrow the Tokugawa, but you'll have to look closely and be creative. Remember, natural disasters were considered heavenly portents and disapproval of the Tokugawa.
As tonsofun pointed out, there's also the possibility of a policy shift by the Tokugawa without a forced opening. The question here is what benefit the Tokugawa could seek that would make such a drastic policy shift (and ideology shift) seem attractive. What could drive the Tokugawa to open the country up to danger?