Make a solid porter, and add spiced chai (or just the spice blend if you can find it or want to make your own) to secondary. You could also make a concentrated tea with the spices and add that to taste at bottling. I like adding spices to the boil for a subtle integrated character, and after primary for a clean/distinct spice character. Hope that sends you in the right direction, good luck.
What sort of porter was the base? I wouldn't go with anything too hoppy or roasty.