After seeing several criticisms, complaints, and wishes regarding Battle Forms, which will likely not be fulfilled by Paizo anytime soon, given that the next book will focus on the Slayer and Daredevil, I decided to present the solution given to Magic+ for most of the aforementioned problems, except for the martial shapeshifter issue. This is probably because the book's focus is on magic.
For those unfamiliar, Magic+ is a book written by Team+, a “publishing house that specializes in expanding content for the Pathfinder Second Edition”. This publishing house not only boasts excellent designers, as seen in Magic+ that also features collaborations with well-known writers and designers in the industry who have worked (and generally still work as freelancers) for Paizo, specifically on Pathfinder 2e materials, such as Brite Cheney, Derry Luttrell (Lost Omens Shining Kingdoms), Tony Saunders, and Mike Sayre (surname of Michael Sayre?), Mark Seifter and Linda Zayas-Palmer.
Since the book is neither OGL nor ORC, I will not copy any complete rules here, only make references to parts of them (which will give a good idea of how they work, but are not sufficient to be fully used without the book). If you liked what you're going to see in this post, I highly recommend you buy the book. It is currently sold at Pathfinder Infinity/DrivethruRPG in PDF. Especially if you have complaints about various aspects of how spells and spellcasters work in PF2e, you will probably really like this book.
Magic+, like other Team+ books, is basically a book focused on expanding the magic options in PF2e, especially addressing points that many players complain about, among them battle form spells, where it attempts to solve the problems by creating a new set of spells collectively called Aspect Forms.
The main characteristic of Aspect Forms over the system's standard battle form spells is that they are subdivided into sub-aspects that basically encompass a general characteristic that covers an entire group of creatures. For example:
The Animal Shift spell gives you access to the sub-aspects Aerial Aspect, which is a stat block that covers the general characteristics for interpreting the form of flying animals such as crows, dragonflies, or even a pterodactyl; and Aqueous Aspect, which covers the general characteristics for interpreting the form of aquatic animals such as frogs, sharks, or spinosaurs. Mount Aspect encompasses the general characteristics for interpreting the form of animals that can be ridden, such as horses, elephants, or terrorbirds, including an exception to the rule of riding allies; Predator Aspect encompasses the general characteristics for interpreting the form of predatory animals, such as bears, wolves, and cave worms.
This means that a single spell is sufficient to cover the entire family of a single trait, such as the animal in this case.
The other difference, and the most important in my opinion, is that almost all battle form statistics are based on the caster's Spell DC and Spell Attack instead of fixed values. If your attack bonus is your spell attack bonus, your AC equals your spell DC, your skill DC and bonus equals your spell DC and bonus, even your saving throws are based on your spell DC/bonus, but with extra bonuses and penalties defined by the form. You can still replace your attack and skill bonuses if your character has higher values for them, but this generally only occurs with skills that use your key attribute as a base, since your spell DC/bonus will likely be better. This solves the whole issue of battle form not progressing as the character's level constantly falls behind. Although spell proficiency progression is traditionally slower than martial attack progression and doesn't receive item bonuses, this ends up being beneficial in other aspects like AC, and in the end, most spellcasters become legendary, helping to compensate for this in the end-game.
Another good difference is that all aspect spells normally heighten up to rank 10, with other class characteristics not based on the spell's DC/bonus being based on the spell's current rank, such as damage, temporary HP, and size. Therefore, your aspect never becomes outdated like official battle form spells.
Another interesting additional feature is that many aspect spell forms can cast spells. They have a small spell list similar to the spell lists of common schools/bloodlines, etc., seen in various spellcasting classes, with the limitation that you must choose only 2 of these spells to be cast only once each, one being at least 2 ranks below your current rank, or the other being at least 3 ranks below your current rank, and they can be raised to these ranks if you wish; or you can take a form cantrip in place of one of these spells (usually the lower-rank one), casting it freely as normal, but with the restriction that your DC/plus bonus receives a -2 penalty on this cantrip.
For those who would like to save spell slots, the book also introduces the Aspect Morpher archetype, which basically allows you to cast your aspect spells using focus spells. However, unlike the druid, the feats of this aspect are not as progressive, since any aspect progresses naturally up to level 20, providing feats to broaden your options much more horizontally (providing extra aspect spell options to use with your focus point more for versatility). Furthermore, this archetype is not locked into primal tradition like the druid, thus allowing for less orthodox forms, such as constructs, oozes, and aberrations.
Unfortunately, this archetype is not a good option for those who would like to create a martial Shape Shifter because, in addition to requiring you to be able to cast a first-rank spell, the battle forms are based on your Spell DC/bonus, something a traditional martial archetype also lacks. You can still use it well in a magus, but with the exception of your attack bonus from any skills you have trained, your AC and saving throws will suffer with these options since the progression of your Spell DC/bonus is 2 levels slower than it would be in a full caster, and you will never be legendary in them. A pure martial could also gain access, but this would first require an investment in a spellcaster archetype to unlock 1st-rank spells, which means that a fighter, for example, would only have access to the archetype at level 8 and would also have the problem of the Spell DC/bonus progression at an even greater degree, leaving the battle form with a higher hit rate earlier, but with everything else severely penalized. That's why it's not worth it.