Prospects

Improved statistical methods for detecting directional codon bias
that can handle large amounts of data, ideally only require usage tables

Explore other factors that can influence synonymous codon usage in 
multicellular organisms. Can use an Nc plot, but against different 
variables:
tRNA population bias
mRNA expression and stability
protein abundance
ribosome loading speed of mRNAs
known mutational bias (polymerase or repair?)
Microarray data may provide some of this information.

Many factors contribute to synonymous codon usage bias, and 
understanding their relative influences will further our 
understanding of evolution.