I'd love to be able to do occasional full rescans of my profile. I know this is more taxing on the server, so maybe restrict this to once a year, or even make it something you have to actually pay for. There are 2 reason I can think of for doing this:
1. People who at some point may have cheated to get achievements and want a fresh start. Those people can re-lock their achievements, and do a complete re-scan. We did this for a few people in beta (myself included, as I cheated long before there were achievement communities for Steam, at which time they were pretty much meaningless).
2. To allow people who joined before
we changed how completion percentages are calculated, who'd like a more accurate measure. I've got probably 90% of the achievements for the games that SHOULD be on my record, but I still only have a 10% completion ratio because of all the games I've idled or otherwise played for a few minutes.