I think we need a large, Rolemasteresque table to cover many of the glorious possibilities.
Like, roll a 66 and get either a demon or drummer.
Ok, not a 66, but a botch. One botch it's a demon, two it's a drummer, three the drummer will still be living at home when he starts making aging rolls.
Many pairings will not produce offspring, but many will, even if the partners are unlikely.
Two things of the same kind in a neutral environment will almost always produce offspring of that same kind. There is a tiny possibility for weirdness because who knows what surprises lurk within a lineage? The more a lineage is likely to have something interesting, the more likely a surprise will occur, such as Faerie Blood (so the stories about your mother's grandma and the seven dwarfs are true) or Heroic Blood (you are of the true line of David, Caesar, Simon Magus, whoever) and so on.
A conception or birth in a biased environment can change things, in proportion to the Aura or sympathy.
Dominance of Realm: When realms differ but are of similar magnitude, the offspring's primary realm from least to most likely is Divine, Infernal, Magic, Faerie, but some virtues and flaws might be aligned to the other realm. If one parent is mundane, the offspring is likely to be a mundane heavily aligned with the realm of the other parent. There is a chance that the offspring will have a small Might score instead and be a lesser being of that realm, such as a Demon Child or Naphil; the chance of this happening is in opposite order to the list above.
The offspring of animals is almost always an animal, but if the parents are of different kinds it will be some kind of hybrid or chimera.
The offspring of a human and an animal is likely to be a chimera with aspects of both parents, and is more likely to have Int than Cunning. Aspects can be physical, mental and emotional.
The offspring of a spirit and something other than a spirit is almost never a spirit.
When kinds mix, the dominant kind of the offspring from most to least likely is Human, Animal, Plant, Element, Spirit.
As before, environment matters. Two animals aligned to the magic realm but without might will probably produce the same... but if they happen to be in a +8 magic aura at the time, the offspring might develop Might.
Definitely agree regarding a familiar not needing Might. Also agreed regarding longevity, though I think there are many weird exceptions. I think I disagree regarding fertility. The LR affects Corpus, not Animal. The bond between magus and familiar also does not involve Animal; they share a lifespan because of the bond, but the familiar is unaffected by the LR.