I have no idea how IBM decides what new features to add to Informix, but I do know that we can now be part of the discussion by using the new Request for Enhancement tool (RFE Tool).
I took this for a spin today and I must admit this is a pretty interesting thing that you should check out. Not only can you submit your own requests for new features, you can view what everyone else has suggested and vote for what you want to have added.
There are a lot of good ideas in there and I really hope to see some of these feature requests in later releases.
Take the RFE for a spin today. Submit a request, it is fun.
To see the Informix specific RFEs, search under Brand: Information Management, Product Family: Informix.