Probably been asked for, but some things I would find useful are:
Date Completed field that could be sort-able by a filter, like how Release Date works, so you get a bunch of years to click on instead, letting you see what you've finished each year. We have a last played field and a completed check box, but there's no way to view last played as a filter like release date, and the check box for completion isn't very informative either. Plus last played and date completed aren't necessarily the same, if you just launch a game to check something out, it would update the date and no longer be correct.
Review score, field for metacritic score (or whatever source you prefer - for movies you could use IMBD or TMDB, etc, most media formats have a similar score). Could make this a custom field I suppose, but I'm not exactly sure if the program can interpret the field in to groups like it does with the Star Rating, where you'd be able to filter games by their score.
Star rating: Would like to see an option to have it based out of 10, not 5, so half stars would be an ideal solution. Probably could make it a custom field too, but doubt it would show up as nicely in the details panel like the current stars do.
Hmm... I see some of this has already been put on the to-do list which I just noticed the link to. Well, I suppose it can't hurt to leave the reasoning and details for the fields, in case they are more comprehensive than what was planned.