Okay, let us start with the good. Princess Cadence is pretty awesome and overall the episodes were way beyond anything I expected. Now the difficult problem I am facing. There are now two characters that will be very difficult to pull into "Making Foals" without basically screwing with everything.

Possible solutions:

Princess Cadence is actually a failed attempt from the project and is not a true Alicorn, and due to the lead scientist of the time, the files were sealed and that line of research was discontinued.

This is an alternate universe with no Cadence.

PegaCorn: Natural phenomenon with the appearance of a Alicorn but no immortality which the main reason for the project is to create a permanent new family member.

I think Cadence in general should not be immortal as it would be kind of sad for her to outlive Shining armor, who also poses some problems in integrating him as Twilight already has a brother in Dusk Shine....