Here are some ideas and projects my daughter and I took on one day. Most of it she did.
Fun makeovers with baby onesies. Glue and Stitch.
Here is a suspender and tie idea.
One outfit was a cut out to make a V-neck then stitched stretch bias tape before adding buttons.
Another idea was adding a snap-on bow tie to a white onesie.
We put iron-on bond to the tie to stick to the onesie then stitched the tie.