Well unless you know what you are doing and have it down to a science after several attempts at cross-breeding the same plants, your results will likely be unstable.
You can cross sativa and indica if you want. Your results should theoretically be a plant that is %50 sativa and %50 indica, but your results will be varied. One plant might be 75% sativa and %25 indica, and another plant with the same genetics could be the opposite.
I am no breeder, but if you are interested in making your own strain, I suggest experimenting and smoking, and find the plant you like best and clone her.
Hope that helps! good luck man!