...but I feel comfortable in predicting the next thing every NIDS vendor will roll into their products: full packet logging.
In a nutshell, analysis will be removed one step further from capture. You'll have a tool that logs all packets to disk. Then packet and stream data will be analyzed on disk. It will work like your current IDS except that you can go back and get all of the packets, not just the ones that the IDS alerted on at the time. There are both performance and forensic advantages to doing this. The main drawback is the amount of disk it will take and how you will manage data retention over time. But disk is cheap and security products are expensive and this is something that will make the old seem new again.