"I could easily see a system where it is considered perfectly natural that a young prince or princess should learn some magic, because magic is power, and royalty should have power. Knowing wizardry is also a symbol of having a good education, and princes and princesses would want to have good educations."
Going back and looking at how I portray arcane magic in my setting, this fits. Wizardry is relatively new (only about a century and a half old as of the first modern age), and it's not easy to learn. If you don't start a couple years before becoming a teenager you'll never be good enough not to be dangerous. It's like learning a language, except thick accents can kill you. Learning is, of course, very expensive. Who can afford that but the nobility? And Wizardry is new and powerful, and nobility likes displaying power. So, it may well make sense that nobles and royalty send their children to study the arts of Wizardry. In fact, it could even serve as an alternate place to put younger children who may otherwise have been sent to the priesthood because they can't inherit. Over time, as magical artillery and professional armies change the battlefield so that cavalry charges are considered risky and cavalry, while still the area of the aristocracy and drinker of glory, is a scouting, flanking, pursuit, and mounted infantry force. With the aristocracy loving mages, but also the arts of the sword and horse, the Magus could well be a more common path for a Knight than the heavily armored martial. The Magus can handle this role fine as is, but perhaps some players want something a bit more martial oriented, maybe with the martial/spellcasting balance of the Bloodrager instead of that of the Magus. More options are nice. Also, it would be cool to have access to Orders and spells.
I know I want full BAB, levels 1-4 of arcane spellcasting, Cavalier Orders, and I'm debating between light and medium armor proficiency (it's not getting heavy, and I think medium may or may not go too far). I do not really want horse based abilities (I think overreliance on mounts hurt the Cavalier). Now I need something unique to give it that a Magus or Cavalier doesn't have, but I'm torn as to ideas. Anyone have any?
Also, any ideas for new orders I could write to fit this theme within a generally Magitech (Eberron style) setting?
I like this idea. I tried something like it once, but I could never get it to work the way I wanted.
What I had worked out before I stopped:
1) Light and Medium armor, shields are fine too.
2) No mount. Maybe use a bonded object like a wizard. That would give them an easy to enchant weapon or wondrous item of their choice at early levels. Could be useful.
3) Instead of using outright Cavalier Orders, I made my own 'schools' that were halfway between Cavalier orders and wizard schools. If you have a magitech heavy setting, try having your orders represent ideas of magic that they would practice. For example, the Order of the Shield practices a lot of abjuration magic and gets similar powers.