This is a bit of a thought experiment which I originally posted in the "Wild Mass Guessing" page on Foundation's TVTropes page, but I figured it would be good to bring up here as well:
The first 5 Seldon Crises were: 1 - Balance of Power, 2 - Religion, 3 - Trade Alone, 4 - Foundation & Empire - A Strong General under a Strong Emperor, and 5 - Independent Traders, internal corruption in the expanding Foundation economic hegemony mirroring the same corruption in the Empire. The last point, of course, being disrupted by the Mule.
We don't know if Asimov himself planned out all 10 Crisis (he probably didn't, at least at first), and the LATER Foundations books threw everything out of balance: Foundation's Edge, Foundation & Earth, and the prequels Prelude to Foundation and Forward the Foundation. The non-Asimov books are of course non-canon, but they don't really address the issue either.
In Foundation's Edge, 500 years into the Great Interregnum, we get the 8th Seldon Crisis: now that the "Foundation Federation" includes a full third of the galaxy, there's a push to move the capital from Terminus at the extreme edge of the galaxy, closer to the center. This move is rejected: it would only encourage the Foundation to expand TOO quickly, emboldening them to absorb the rest of the galaxy so fast that it destabilizes (particularly the dense, rich sectors of the Inner provinces of the galaxy, which despite making up one third of the galaxy's volume make up three quarters of its wealth and population). Asimov just skipped over the 5th and 6th crises, and we never learn about them. At a rough guess, perhaps the 5th or 6th Crisis is whatever led "the Foundation" to reform into the "Foundation Federation"? From comments made by the Second Foundation's First Speaker in the same book, the goal is that the Second Empire will be a "Federated Empire" with more control shared by the provinces, not the overcentralization that led to the decline of the First Empire. A separate question, is the 9th and 10th Crises, which may or may not happen after the revelations about Galaxia at the end of Foundation and Empire. As Seldon points out, why would Galaxia need an Encyclopedia Galactica? Even though we know it is later printed.
As explained at the end of Foundation & Empire, Psychohistory has one major, unstated limitation: it's designed to work on a galaxy filled with *humans*. Based on their psychological patterns. Even robots technically count as a human action, or perhaps, their minds are similar enough to humans or at least predictable enough that it can account for them. But there are two scenarios it never accoutned for: 1 - Sapient alien races develop very rarely, to the point humans are the only one in the Milky Way Galaxy, and most other galaxies probably have only one or none. Humans were never very inclined to spread the vast distant to other galaxies, content to fight other humans for control of the Milky Way. But statistically, there must be some galaxy out there that by luck of the draw actually got two (or more) major sapient races in it. They would fall into inevitable conflict, ending only when one was the victor - leading to an expansionist mindset, encouraging them to be *inter-galactic* conquerors, eventually spreading to the Milky Way. And psychohistory cannot predict their actions.
2 - Trans-humans. Genetic engineering advancing to the point that trans-humans are so altered from the human baseline that their psychology becomes for all intents and purposes "alien", unpredictable by psychohistory. Such as with the Solarians, the isolated last surviving Spacers who have been artificially evolving on their own for over 10,000 years.
Realizing the limits of Psychohistory, R. Daneel Olivaw created Gaia - a planet-wide hive-mind, hoping it could one day grow into "Galaxia" - Gaia on a galactic scale.
But throwing a wrench in plans here is that....Seldon knew about Galaxia. As we find in Prelude to Foundation. This leads to several logical possibilities: 1 - The original "Seldon Plan" assumes it will be chosen instead of Gaia/Galaxia, or that Galaxia will fail somehow. In which case the final two crises don't really take it into consideration. This version of the Plan also strictly assumes that no Aliens or Transhumans appear to mess it up.
2 - Given that Seldon knew about Gaia, it's possible that the final two "Crises" actually anticipated and incorporated Gaia into its chain of events. Leading to two possibilities:
2A - Seldon intended Gaia to complement his Plan.
2B - Seldon opposed Gaia. He was willing to keep it around as a fallback option, but if the Foundation succeeded in creating a Second Empire without Gaia's intervention, he'd want a plan in place to defeat it.
3 - It's possible that the final two crises actually do deal with Aliens or Transhumans. Psychohistory might not be able to predict their actions - but by the same logic, Psychohistory didn't predict the Mule either. A mutant with mentallic powers (later revealed to be a renegade from Gaia). Are not the Second Foundationers, with their mentallic powers, essentially Transhumans already? Only a matter of degree removed from Solarians? Yet while Psychohistory couldn't "predict" the Mule, as such, Seldon's "Plan" went beyond mere predictions, with anticipated fallbacks and safeguards specifically against unpredictable events. In short, it's possible that Hari Seldon himself already predicted everything that the characters discuss in the final chapter of Foundation & Earth, and had already incorporated all of these possibilities into his plan!