I've always considered Orcs to be, in a physiological sense, more similar to Humans than most species. They're the only race other than Elves that are capable of producing fertile offspring with Humans in 5E, after all.
I have two responses to this, so I’ll split them up:
1) On Hybrids
Half-___s always being half-human has always been something that’s weirded me out about D&D. There comes an implication that there’s something special about humans that makes them particularly capable of successfully inter-breeding with other creatures, which is… an odd “ability” to give just to humans imo.
I think it also puts a strange dampener on creativity. Why can’t I be a Half-Halfling (half-human/half-halfling)? Or something not human at all, like a Dwelf (dwarf/elf)? It’s my game’s setting, who is WotC to tell me who can and can’t have successful progeny?
Some editions kind of skirt this issue by never explicitly saying that the other half is half-human, but I find that’s self-defeating, because really it just comes down to half your lineage not really mattering. A half-human/half-elf ends up having the same traits as a half-dwarf/half-elf. In every edition of D&D I’ve played, half-elves in particular have always gotten the short end of the stick – getting some of the features elves do and then… nothing else important, really. What does the other side of my lineage mean for me?
This is why Project TOF will have way more comprehensive hybrid-species rules.
2) On Orcs / Species Weeks
So is it a part of your consideration that orcs, like humans, are mammals? That they reproduce in roughly the same way? What physiological traits in particular do you think make orcs so akin to humans?
Something I’ve noticed about a majority of responses to these Species Weeks has been people using one fantasy species as a reference point for describing another. These kinds of responses are… generally unhelpful, because what do you think the other species you’re comparing them to is like? Saying “X is like Y but bigger” only helps if I know what you think Y is!
I get not everyone wants to write me out a page, or even paragraph-long response. In fact, I don’t expect anybody to. But what I would like are some grounded details. For example, last week I got some feedback from people who envision gnomes having sharp teeth, or eyes like an owl’s, or mostly subsisting off of nuts. These are helpful in giving me an idea of what people think a gnome is!
What’s an orc’s anatomy like? How tall are they on average? What are they built to eat? What marks them as a distinct species, not just a human in cosplay?