There's one NPC I'd be interested in for my own saga. I could design him (or her, up to you) myself, and indeed I was doing so, but upon seeing this thread I realize I'd be happier with something designed outside of my own troupe.
These are the design constraints.
a) A Tremere holding his own sigil.
b) Specializing in Muto, Intellego, Animal and Corpus (his "tines" as per HoHTL). Up to you to decide what kind of concept works well with those four Arts. My idea was a lineage descended from shapeshifters and beastwhisperers of old; thus a Hermetic shapechanger and spy. Then, I'd envisioned a character with a mercurial, unpredictable temperament to match. But it's not set in stone - only the Arts are.
c) Absolutely average at Certamen (for a Tremere who does not specialize in certamen; note that only few do according to HoH:TL).
d) With one gauntleted apprentice. The apprentice ended up "average" (i.e. built by the ArM5 rules without particular Virtues or Flaws related to training) and with a normal Gift.
e) The apprentice was gauntleted 7-8 years before any snapshot of the NPC (who will pop into my story some 7 years after the saga started - note that the saga does not start in 1220). The apprentice would have been taken when the NPC was of an age you'd consider typical for taking one's first apprentice, or up to a decade later. You don't have to design the apprentice, we already have that!
This concept is different from most of the other concepts in the thread in that it does not try to be something particularly creative, but instead arose organically from the needs of a real saga. For the same reason, I'd rather have an NPC who feels "real" rather than particularly thought-provoking.
By the way, if anyone else wants to try one's hand with such a magus/maga too, I'd be even happier 