Hi Anbu,
Which kind of plugin are you making? The previous answer hit on just one of the multitude of areas that a plugin could service. That, of course, being the display of market products using your affiliate links. However, there is a whole range of services that include the sales side of things. This would include everything from sales buttons, thank-you pages, electronic downloads, and even membership site management.
Three areas I personally think are missing (just from my experience, I'd love to hear if anyone knows any that exist)
-- Low Cost Membership Site Handling (Currently, quality ones run from $99 for a single site to $299 for multi-site). A decent, lower-cost plug-in would be great for newer marketers and I bet you'd get a lot of business if you had the features.
-- Affiliate Tools Page: This is the page that you refer people who want to sell your products. - I realize some people may laugh at me, but when I first got into marketing, I had to learn to create my own. there are a lot of features that if you are not a web developer, aren't exactly easy to create. Allow affiliates to register, display appropriate (customized) affiliate links for each of them, manage banner ads (with copy-paste code for the affiliate), e-mail swipe management, etc.
-- Integrated URL Links: There are tools that just do this (pretty little links...). But, I would like one that is integrated into the affiliate tools we mentioned above. Ie, it can auto-generate masked URL's for each of your products, and auto-generate a link to those products for each registered affiliate that has registered as part of your affiliate program.
The ability to select your CB product, and auto-template the last two features mentioned would be a huge timesaver for people looking to step-up their marketing of their own products.
Just my two cents...
~ Delana