Yes, I'm using three different versions of these spells depending on the part that is changed.
Three sense-enhancing enchantments, Base 2, +1 Touch, +1 Diameter.
Two body-enhancing effects, one for the main body and another one for the legs, Base 10, +1 Touch, +1 Diameter, +1 Part.
Two effects that give extra body parts to fly/dive with, Base 20, +1 Touch, +1 Diameter, +1 Part.
They'd all be invested with unlimited uses and environmental trigger set to recast the effect as soon as it ends, for 13 extra levels. Total levels would be 17 for the first version, 38 for the second, 48 for the third.
I was thinking also about the possibility of having them rely on an arcane connection to decide what animal to transform into. That way you could update the effects after finding animals with better characteristics, without having to redesign the device. So for example, to get the device to give you owl eyes, bat ears and hound nose, you'd have to attach something that belonged to each animal to it.
If the effects are to be recast every Diameter, wouldn't the magus need to supress his Parma equally often in 5e, unless the effects have sufficient Penetration?
I would go as far as having to have not only a piece of animal, but the exact piece you want to acquire the characteristic from. The spell cannot know if you want the eyesight or the nose of a dog if your are attaching a dog's tail to the item.
But you should gage carefully if you want to have "flexible" spells in your saga. Normally, spells have very specific effect and do not allow much flexibility. A Pilum of Fire is always a trait of fire, the magus cannot decide that it is a small ball, or can make curve trajectory or even change colour.
If you allow a spell with build-in flexibility, expect to have more.
I was going to suggest considering a MuVi for a significant change of the spell effect, but it is clearly stated in the Muto vim guideline "Note that a spell invented according to one of the guidelines below will change another spell in a specific way: it is not possible to invent a single spell which changes another spell however the caster wants".
Hmm, to be honest I didn't even think about that. Our storyguide doesn't really bother too much about the concept of Parma suppresion, he usually rolls with "if you'd want to get hit by that spell, then you just do". In any case I like to play by the rules unless they contradict some HR. Going with D:Concentration would be 2 levels higher since I'd swap the environmental trigger for maintaining concentration, but I could also reduce their frequency to 24 uses per day.
Personally, I miss the Constant frequency from the old edition. The fact that you could use that and D:Momentary made enchanting kinda broken, specially for a Verditius, since it made enchanting actually cheaper in many cases, but now you gotta mess with too much crap to get the same effect for a truckload of extra levels. I also think that they decided on a too convoluted way to create a "constant effect" setup.
I'd propose an enchantment-only duration, some sort of "D:Enchantment". It'd pretty much be a D:Ring in which the spell depends on the existence of the device itself, instead of the circle. As long as the target is valid and stays within range, and the device is not disenchanted or destroyed, then it can potentially last forever. I'd say it should be of equal magnitude as D:Concentration, since this one doesn't involve the possibility of changing the spell on the fly. That would make this sort of spells usually 5 levels cheaper than one that maintains concentration, which I think it's fair.
Then you can tweak the frequency and other effect modifications to get stuff done. R:Touch, T:Individual and ulimited uses would make a device that is always active as long as it's worn. A single use would be enough if you want to create a device that has an everlasting effect on itself, you just activate it when ready and that's all. Now that I think about it, you could use this method to create a charged item that ran an effect forever, since all you need is the initial use. Or you could reason that the enchantment no longer exists after discharged, and therefore this duration would basically end as soon as it's triggered.
Well, you could tailor each effect for each specific body part that is supposed to change. The spell that targets the eyes will always deal with eyes, so the animal's focus would be there simply to tell the spell which animal's eyes. Otherwise it'd be kinda tricky if you also need to carry a bear's torso and a pair of panther's legs, lol.
But you can't do that with a Pilum of Fire because the spell was designed specifically to look like that. Nothing says that you can't make fire spell that throws a projectile with variable shape, color and trajectory. In this particular case, the rules even specify that all of those traits are purely cosmetic effects, since you don't even need to aim. You'd simply make a CrIg(Im) spell, and the requisite would even be free.
I understand that the rules allow to design spells that have some degree of flexibility, although that usually reflects on a higher level. I would assume that a spell that lets you turn into any animal as long as you have a focus would be greater than one that was designed with a particular animal in mind.
I don't think that's a bad thing, if properly justified. There are already examples in the books, after all.