This was originally posted in September. Have you found a solution to work with? The forecasting tool in Tableau is documented here - Forecast Options Dialog Box - Tableau. There are several ideas for improvements to forecasting in the @ideas area of the community forums. You may wish to add a vote to the ideas that would best work for you or create your own idea for improvements.
Many users are doing more complicated forecasting analysis today with the use of integrations to R.
I hope this helps,