Pastel works well with just about any morph. I would recommend any of the following:
- Spider
- Fire
- Yellowbelly
- Black Pastel
- Lesser
- Mojave
- Butter
- Another Pastel
- Enchi
- Pinstripe
All of these are co-dominate/dominate morphs so you should be able to produce something cool on the first breeding.
For a more long term project, here are some recessive morphs. These will take multiple breedings to get something new (visually) but can really produce some cool snakes:
- Pied
- Clown
- Axanthic
- Hypo