I really think you should allow members of different races to mate… there can be several reasons for it, from personal desire to wanting to reverse a race’s imminent extinction. FTO had some problems in this area, but I think it would be very easy to avoid them. It broke down to (I think) two: Impossible crossbreeds and unfortunate combinations.
Impossible crossbreeds: Mostly a relative size issue I think, such as the giant dragon mating with a pixie. Personally I think it could be easily ‘possibled’ in RP, like if one used magic to temporarily equalize their sizes, but if we want to avoid it all together, just assign a size rating number to the races, and disallow matings between creatures more than 1 size different.
Unfortunate combinations: I forget what the exact combination was in FTO, merfolk crossed with human or elf I think, but the effect was crossbred children who had neither parent’s mobility… couldn’t walk on land, or swim, so they couldn’t leave their birth square. But this would be solved by what I propose below:
I wanted to suggest a very simple traits system (or less simple if you want to program more options/layers) that would have 3 components:
Trait A: Racial: Defines dominant parent, setting base race-identity and default racial appearance, and any critical race-specific traits such as movement type (if you’re primarily a mermaid you will get to swim, a pixie to fly, an elf to walk, etc.) or vision (if for example, light is a thing in the game, an underground-dwelling race would get to see in the dark). This makes sure the character is not inherently helpless in some way.
Trait B: Inherited: Random chance to inherit one or more of the other parent’s traits. 50% base chance to get either parent’s racial stat-bonus and skill bonus (separate checks). A much smaller chance to get both parents’ bonus, or additional features such as movement type (eg: an elf who can also swim, aquaman like) vision, etc. Includes chance to inherit special traits (below).
Trait C: Random/Destiny: I think there should be small chances for anyone to have a special trait that just occurs at random. The magic powers some had in FTO are a good example. Unusual stat boosts (like pippi longstocking’s strength), unusual defenses (I always thought there should be defensive combat magic too - faster healing or regeneration in combat, blinding flashes or telekinetic force to make an attack miss sometimes, etc), gills to allow foraging underwater, extra foraging luck or quantity, etc.
Anyway, since trait A would always give a character of mixed parentage a defined racial base and its critical elements, you’d never get someone being stuck and helpless like in FTO.
For trait B you could also have possible inheritance of cosmetic appearance traits - if the features described in the default racial description for instance were standardized (i.e. A; members of this race have skin that ranges from a medium tan to the deepest black B: typically range from 5 to 6 feet in height, etc) then you could chance inherit one or more from the non-dominant parent, and have their coloration, facial hair, etc. A dwarf-bearded mermaid, a human-height halfling, etc.
It probably sounds more complicated than it would be… really it would just involve comparing a string of trait values in the parent characters’ files, and making some rolls to set the same string of values in the new character. It could be as simple as only determining a racial base and ensuring the critical cabapilities of that race (swimming, cold protection for ice creatures, etc) or what I described above, or a more complex system that maintains %'s and could cause a trait from a ancestor generations back to come out by surprise (like an elf with a cat’s tail). You’d just have to decide what system to make. But I’d really like to see something beyond “you must be the same race to mate” as a rule.