All the onesie patterns I've come across and tried always turn out looking more like for preemies than for a term baby and I've even gone up as much as 3 hook sizes to a K before it looked like it would even fit a newborn. I know I'm not crocheting that tightly since I'm keeping my tension as loose as I dare.
I decided to try my hand at making a sleeper. It seems to be going pretty well, I wish I had a baby sized doll to see, right now I'm using a teddy bear about baby size just for reference so I at least know it's big enough. I'm writing the pattern out as I go so maybe I can share it when I'm done if it turns out right. It'll be short sleeved like the diaper shirts and onesies but it could also be made long sleeved quite easily.
I'll keep you updated on my progress and pictures will be forthcoming if it does what I hope it does.
Wish me luck, it's my first try at my own design.
If this works I might adapt it to a onesie and diaper shirt.