Not a ref, but I would imagine that you can’t add anything like a new realm - spring, summer, autumn, winter, day, night. However, there are plenty of “techniques” - The most common magical techniques are; Rune magic, Dramaturgy, Astronomancy and the Music of the spheres. You can read about them on profounddecisions.co.uk/empire-wiki/Rituals under the “performing a ritual” heading.
Also taken from that page: It is possible to create a ritual at an event that is not part of the existing library of known Imperial rituals. In this instance you will need to consult with a referee who will tell you what realm the ritual effect is and how much mana is required to perform the ritual. Spontaneous rituals may vary in cost from one event to another. If you learn to master a ritual that you have created then the cost of that ritual is fixed from that point onwards.
New “techniques” how? Like astronomancy or dramaturgy or the Music of the Spheres? Just make them up and write an IC book on it. I mean we have the “Upwold Method” for farming rituals which involves everyone bringing some produce and the Landskeepers trying to one-up each other on the fertility innuendos. I should probably codify that in a srs book.
A good place to start is Hearth Magic, in my opinion, which is why the Upwold Method of Farm Enchantment sounds great to me. Hearth Magic is meant to suggest magical rules that have the ubiquity of gravity in the Empire world - although they are not as reliable as gravity is. Symbolism is great, again in my opinion, as is internal consistency between your rituals. Start with a premise - that the stars above represent patterns of power, or that dramaturgical roles create powerful symbolic conduits that let you use magic, or that a coven that creates a powerful emotional atmosphere can draw power from the concepts of the REalms themselves … then work out how to make that into a ritual.
I can’t speak for anyone else but I’m reasonably open minded* when it comes to magical roleplaying that is cool and adds to the game (and enthusiasm goes a long way towards making something add to the game). There’s also inspiration for “stuff that works” on some of the foreigner pages - and in the Common Elements section of each ritual.
If I have a prejudice it’s that I like rituals that involve doing something rather than just saying something and waving your hands. Set fire to stuff, drink stuff, pass stuff round, dance, play music, expound extemporaneously on the nature of the Claw and take on the aspect of it’s Tulpas, divide your ritual into thirds and each group make one of the calls of the Chimera; read select passages from a book while other peoplec annoint the target’s knights with rare oil and cinnamon; gird each target with a favour that is IC woven from iridescent butterfly cocoons and dyed with an exotic bulb that grows only on the graves of lovers who died of a broken heart …
As long as it doesn’t involve the Charm of Making from Excalibur (although that said I would trade a freddo to the first group to do an entire Empire ritual in Welsh while keeping it completely IC).