Actually, in regards to half-elf/half-ogres, such creatures do in fact exist in D&D. Sometime during the early to mid 90's, in an installment of 'The Adventures of the Princess Ark' (a serial about the exploration of the far corners of the world of Mystara in a flying ship), the crew of the Ark ran into a culture of halfbreed elf/ogres called N'djadtwa that lived on Davania, the planet's southern continent. I don't remember the details, but I believe they were said to be a little taller than humans but about the same build, with light brown skin; their facial features were rather ugly, like ogres, but with an elf's high cheekbones and pointed ears.
As far as my personal setting goes, I tend to allow the following crossbreeds:
- Half-human/half-elf: same as PH
- Half-human/half-orc: same as PH
- Half-human/half-ogre: same as in Savage Species
- Half-human/half-dwarf: looks like a skinny dwarf about elf height, or a short, stocky human
- Half-human/half-neandethal: pretty much just a short, ugly human (neanderthals IMC are civilized members of society)
- Half-dwarf/half-gnome 'gully dwarves': same as those from Dragonlance: short and dirty, and limited in intelligence, found in dwarven ghettoes
- Half-dwarf/half-neanderthal: pretty much the same as a human/dwarf but stockier and uglier
- Half-gnome/half-halfling 'low gnomes': smaller than gnomes and either more heavyset (if the halfling parent was a 2E-style hairfoot) or skinnier (if the halfling parent was a 3E-style lightfoot; I make use of both)
- Half-orc/half-ogre: if the mother was an orc, it's an orog (no connection to the orogs in Races of Faerun); if the mother was an ogre, it's an ogrillon. Both can be found in the Creature Catalog
- Orc/ogre-dwarf/gnome/halfling crosses or others might be possible, but are very rare because of the difference in size and temperament of the races
- Half-goblin/half-hobgoblin: a little shorter than elves, pretty much look like tall, muscular goblins with sharper features
- Trolls are magical in nature and can breed with just about anything the right size
I also have a few races that are descended from halfbreeds but have over time become 'pure' races: derros (degenerate human/dwarves that actually came to my world via portals from the World of Greyhawk, their homeworld), sundered and sea dwarves (surface-dwelling dwarves with a little human blood), and a few cultures of pureblood half-elves.
These decisions are based pretty much on what branch of the humanoid family the race in question belongs to: 'hominds' include humans, neanderthals, dwarves, gnomes, halflings, orcs, and ogres (elves are actually magical beings created directly by Corellon that can only breed with humans); and 'goblinoids' include goblins, hobgoblins, bugbears, norkers (goblinoids from the 2E Greyhawk MC), and erlkings (forest goblins from Dragon Magazine), but only goblins and hobgoblins are known to be interfertile.
In regards to non-hominid or goblinoid humanoids, I pretty much don't go for anything, with the following exceptions: lizardfolk and troglodytes can crossbreed to make trens (see the CC), lupins (dog-folk stolen from Mystara) can crossbreed with wolvenfolk (humanoid wolves) to make 'demi-wolvenfolk' for lack of a better name, and various subraces of rakastas (cat-folk also lifted from Mystara) can crossbreed (specifically: cat/lynx, lion/tiger, and lion/leopard).
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