I'd like the ability to bucket people into a variation based on URL query string.
Here's my example. I'd like to run an experiment that actually starts with an A/B test through my email marketing system. I want to send two emails that run different special offers. Links from each email would have corresponding campaign codes in the query string.
I'd like to use the value of that campaign parameter to show the user the corresponding special offer.
That's just a specific example though. You can imagine running other campaigns - for example through Google adwords, touting different offers or similar.
Being able to plug in to the query string in the URL would allow me to run integrated tests spanning multiple systems.