Cassandra from Rosy Lou shares a free pattern and tutorial over on her blog, Cass Can Sew, for making a vintage style baby romper. It has a bib front, bloomer bottoms, and crossover back straps. The pattern is in a size 12-18 months. If you need a larger or smaller pattern, you could probaby adjust it using a similarly shaped garment as a sizing guide. Get the pattern and tutorial.
[photo from Cass Can Sew]