For me a fashion designer career would be much more interesting than a model career, though I'd like to see both. A freelance career would be one option, but a semi-active or active career as a fashion designer would be nice too. Involve trendsetter career so you can work together with a trendsetter to sell your clothes, or actively choose to promote a designer if you are a trendsetter. Or have trendsetters review fashion just like critics review food or performances.
I would also want cross pack integration:
* GTW: Start your own fashion store, dress mannequins in your looks and have them sold there. Or start a sewing business where costumers can come to get their clothes repaired.
* DU: A fashion education that teaches you how to design and sew. Could lead to the trend setter career or the fashion designer career.
* GF: Build fame as a model or fashion designer. The more famous the more you get paid, the more people wear your clothes.
* Moschino: Integrate the photo career so that you can have real fashion shoots with the clothes you've designed, or as mentioned above, with the model career.
I would also love a hidden venue for fashion shows (similar to the movie studio in GF or the police station or hospital in GTW) that you could decorate based on the fashion show and then host it. I would also love it if EA used that venue for their own "fashion shows" to introduce new CAS content.