I ran this by one the texture artists, to see if it was possible. I know it's a pretty big job, but I'm willing to help in any way I can if y'all like it. The Patronus Charm it'self won't be that hard to code I'm sure, something like a protection spell only blue. As for it's functionality again'st dementors, perhaps that will have to wait until dementors themselves are added Though there are a couple other functions that the charm could have until then: similar to Wingardium or the way Vicis comes back to wherever your wand is on the re-bound, perhaps you could control it's movements. Even send it somewhere at a distance, or set it to walk to a certain coordinators and deliver a message or item or something. As for how on earth you could give everyone a custom patronus: Apply for a license just like apperation, have 'em take a simple personality test or something, then just add another Patronus to the config. Although, this would take a personal texture for each Patronus. Re-texturing animals isn't that difficult, and it would give the texture artists (who I must say are really good at their job) something to do. There's a method, using MCPatcher, that you can add any amount of textures to the same item, that change with the lore/name of the item/entity. The only problem with using this method is that MCpatcher is something each individual player must run on their computers, though it's really easy to just download and run the .exe. Shouldn't be a problem for anyone. This all makes it possible to re-texture the same cow 30 times and end up with 30 patronuses, that are all cows at heart. So we don't have a ton of 1st years applying for these, maybe it could be y5/6 and up or something, again like apperating. If you want to take the lore a step further: Corperal and Incorperal patronuses. The incorperal is just a shield that blocks dementors, just like any other protection spell. Everyone could probably have this. Corperal patronuses are the full bodied spirit guardians, which you would apply for.
On to the Animagus aspect!
If 'ya go with the Patronus idea, you're already having people apply for animals with custom skins. So why not have a separate application for Animagi? It could very easily be accomplished with libsdisguises. Perhaps before adding any sort of Patronus or Animagi system, a simple 'Magical Creatures' system wouldn't hurt. Thestrals, hippogriphs, pheonixes, etc. Finally a chance to make use of that care of magical creatures classroom.
Side note: EffectLib has some incredible, quite stunning particle effects it can do with just as much lag/fps drop as the sphere effect. Maybe throw in a vortex, wave, or warp sometime
I suspect quite a bit of this is planned, but I'm suggesting it anyways XD
Thank's for taking the time to read through this.