Science Fantasy
While Tékumel is what I have called a "secret science fiction" setting, the extent to which published materials lean into this varies. For my part, I lean into it heavily. Indeed, that's a huge part of the appeal of Tékumel: I like "fantasy" settings where all their fantastical elements are examples of Clarke's Third Law ("Any sufficiently advanced technology is indistinguishable from magic."). This isn't really a change to "standard" Tékumel, but it colors my presentation of everything from magic (and "magic" items) to demons and the gods. This has allowed me to get a better handle on how all the setting's various parts work together and given me lots of ideas for developments in the campaign. Doing so also inadvertently gave birth to the sha-Arthan setting I've been working on for almost four years now.
Magic
For the most part, I stick to the presentation of magic and spells in Empire of the Petal Throne, which is much closer to what's found in OD&D than in later, more "authentic" Tékumel materials, like Swords & Glory. However, as I mentioned in the previous paragraph, I look on magic as "sufficiently advanced technology," even spellcasting. This means that I see a strong connection between spells and magic items, since they both harness the same forces, all of which are explicable by far future science. In this way, I've made it possible for spells to be used as energy sources to reactivate uncharged or even damaged magic items, something that's proved important on several occasions in the campaign.
Demons and the Gods
In a similar way, I views the various demon races of the Planes Beyond and even the gods themselves as highly advanced beings akin to those seen in older science fiction like Star Trek. They're "divine" or "demonic" only in an analogical sense, as humans and other more limited beings attempt to understand their nature and truly alien thought processes. I've also muddied the waters somewhat with the introduction of advanced artificial minds that are themselves effectively gods – and indeed have been mistaken as such by humans. My Tékumel is a place that's littered with science fictional elements dressed up in fantasy garb.
The Pariah Gods
Speaking of the gods, we have the Pariah Gods, a trio of deities introduced into post-EPT Tékumel as antagonistic beings more akin to Lovecraftian entities than those of the pantheon of Pavár. In published Tékumel, the Pariah Gods exist on the fringes of the setting. I've made them much more central, particularly the god known only as The One Other, who not only played a role in the imprisonment of the god Ksárul but was also a catalyst behind the founding of Tsolyánu itself. There are additional changes I've made, but I can't say much more about them here, since the House of Worms campaign is not yet over and I don't want to spoil anything for my players ...
Parallel Worlds, Time Travel, and the College
Furthering my science fictional emphasis, I've made much use of parallel versions of Tékumel, time travel (or at least asynchronous temporality), and the Undying Wizards of the College of the End of Time. None of these things is central to my version of Tékumel but they have roles to play. For example, Toneshkéthu, a student at the College, has been a longstanding ally of the characters. Because she exists in the far future of Tékumel, she often appears "out of sequence" from their perspective, remembering things that haven't yet happened and being unaware of events in which she (or a version of her) actually participated.
History
Speaking of the passage of time, the societies of Tékumel as presented in published materials are old – unbelievably so in my opinion. There is recorded history stretching back more than 10,000 years and I simply can't believe that. Consequently, my version of Tékumel is old but not that old, with suggestions to the contrary simply being rhetorical/poetic exaggerations for effect.
Tsolyánu
The titular Empire of the Petal Throne is presented as if it's much more stable and monolithic than I can accept. Consequently, I've presented Tsolyánu and much more varied and prone to periods of rebellion and even anarchy. Customs and traditions vary from city to city and region to region, even to the point where Tsolyáni from one part of the Empire feel almost like foreigners in another.
Salarvyá and Yán Kór
I've made some changes to two of Tsolyánu's neighboring empires. In the case of Salarvyá, I made it an elective monarchy that periodically convulses with chaos as the time to elect a new king draws near rather than a kingdom ruled by the same dynasty for untold thousands of years. Likewise, Yán Kór is presented in published materials as a major rival of Tsolyánu, thanks to the determination of its leader, Baron Ald. I've opted instead to make it a weak confederation of city-states that's more a threat to itself than to anyone else. Consequently, the war with Yán Kór that occurred in "prime" Tékumel never did in mine.
Heirs to the Petal Throne
I included all the heirs mentioned in the original Empire of the Petal Throne, but almost none of those introduced in later materials. In particular, I dispensed with Mirusíya, whose revelation and subsequent elevation to the Petal Throne in official Tékumel never sat well with me for a number of reasons. Instead, I introduced my own additional heirs, as well as my own spin on the existing heirs.
As you can see, my personal Tékumel doesn't deviate too much from what's found in published materials. It's more a matter of emphasis, which allows me to put my own spin on certain aspects of it. This, in turn, allows me to shape it a setting conducive to the kind of adventures and situations that play to my own interests and strengths as a referee. I think it's worked very well over the course of the ten years we've been playing House of Worms. That said, I will be glad when the campaign is done at last. I've inhabited Tékumel for a long time now and am looking forward to he opportunity to explore a new setting with my players.