Hopefully, this is a little less broad. Apologies if it isn't! I'm rather new here, so I'm not used to this yet. Oops. Also, apologies for the length - I'm trying to explain as much as I can.
To pull some of the stuff I know already; Human babies are born underdeveloped compared with other primates. Our brains are less than 30 percent their adult size at birth, compared with around 40 percent for chimpanzees, our closest living ape relative. It would take a gestation length of 18 to 21 months instead of nine months for human babies' brains to reach that level of development.
I'm not entirely sure why humans cut off so soon, past something to do with their pelvis and standing upright. The species I'm developing, however, needs to be more active from the get go - not that I'm saying they need to give birth to teenaged kids or anything, just that here are more areas in the brain that need to develop. I'm aware the kids are likelier to be bigger as a result when they are born, considering they need extra time to develop.
If a species changes, usually, it needs a reason to change. In the case of my species, the brain needs to develop further for the child's survival. This has to do partly to account for some changes from the 'human' setting. If, uh, that makes sense. I'm kind of using humans as a base here. (Evolved parallel to, branched off at one stage.)
The brain, for one, needs to possibly account for any changes as a result of heightened senses, hearing and smell in particular. This particular one is brought on due to the fact that the planet is rather cluttered - this has bred a large amount of species who rely on stealthier tactics. Though the world is varied, I picture something perhaps mountainous and/or heavily forested for the general area this species evolved in. (Leaning more towards 'rainforest' as opposed to plain 'ol forest.) They've spread out pretty recently - but they've still adapted to this particular environment.
Another change is magic; like I brought up in an earlier question that I worded a bit awfully, it highly impacts this species (and many others native to the planet), to the point where it has interwoven into their evolutionary process. It's second nature to them, like how humans have evolved for... hunting? Help, words. Take the wizards from Harry Potter as an example. One needs to be born with the magical ability to actually go around wielding magic. Wizards can, but muggles (normal humans) cannot. Occasionally people are born who can't use his magic, regardless of bloodline (squibs). The failure to develop this magical ability for my race, as a result could be due to premature birth or birth defects.
I'm aiming for the gestation period to last as long as 15 to 21 months, if required - it's likely I'll be adding or adjusting other areas of brain development later. I can handwave some of it, but not everything. The race was more solidary many millions of years ago, but over time developed into something resembling larger flocks. This is only strengthened further once longer gestation periods become a thing, considering the one carrying the baby isn't going to be at her peak if/when predators come.
The senses and inclination to magic are what I'd consider defense mechanisms. Natural ways the body has evolved to let them survive. Kind of like long necks on giraffes, though my race doesn't go around hitting people with their necks... well, not yet, anyway.
I'm planning to keep the race bipedal, if possible, or perhaps being in some process of evolving away from the whole four legged thing. Vaguely humanoid. The organs and stuff on the other hand are basically free range if it gets the desired results though.
I'm not really sure what else to add, but I'll keep an eye out so I can edit accordingly. Sorry for the brain-vomit and I appreciate any help.