The problem (as I and others see it): More expensive mounts can become a liability if the owner doesn't have access to a collar. What this results in is people with rare (ex. bazaar) mounts not wanting to risk them by taking them places, and sometimes simply leaving them in stables or places where people won't see them. They become more of a 'yes I own X' as opposed to 'look at my horse, my horse is amazing'. With changes to mount killing (both novices killing mounts and suiciding, as well as mount killing in general) these rare mounts could become more useful, without the large cost of purchasing a collar for the mount.
A) All mounts respawn on a long timer, perhaps 24 hours? The Collar would change to drastically reduce this time (2-4 hours instead?) so that the mount is available more often, but it isn't required for the mount to be worth bringing out. A downside to this is it greatly devalues the collars as is.
Move all Riding skills down, new Transcendant riding skill that allows you to train one mount at a time to flee from battle. This would generally be whatever one mount you have out with you, and if that mount dies, instead it flees the battle, and is forced to recover in the stable for a set period of time. During this time, you can't train another mount.
C) Some gold-based training or item you can attach to a mount that causes it to revive once, after a long delay. This would have the effect of basically being a cheap gold-based collar, that would eventually be less efficient than a collar if your mount is dying all the time, but more efficient if you take proper care of it.
These are just some off the top of my head solutions, and I'm sure there are plenty of others that people could think up. I want to see more of the rare mounts that people own out and about because they're not a liability, they're a cool thing to show off.
Cascades of quicksilver light streak across the firmament as the celestial voice of Ourania intones, "Oh Jarrod..."