Here's some simple PJ pants I made using the trousers from Simplicity 1668.
I made quite a few alterations.
1. Raised the back waist slightly.
2. Changed the back crotch to be more of an L then a gradual slope and slightly extended the back crotch length.
3. Removed the grown on elastic casing, which I replaced with a waistband piece.
4. Added buttonholes at centre front of waistband to allow drawstrings to come through.
5. Instead of the full elastic waistband I cut a slightly shorter piece of elastic and added drawstrings to each end. This gives the stretch of elastic, but the adjustability of drawstrings and is my preferred finish for PJ pants.
6. Shortened the leg length by 11 inches (28cm) to give a 3/4 length pant leg (as these are summer PJs)
7. Added a lace trim to the hems.