Firstly, I've always liked the idea of an open source photo gallery which is simple, fast and clean. Plogger is all that and more. But there are a few things I wish to see in Plogger in the future releases. (Shouldn't be too much of an asking from people who have created a really good script, I feel :-))
Anyways, for starters, I wish there was a rating system where viewers could rate pictures and the database holds the ratings. Based on this rating, you could show "Most Popular", "Most Rated" Pictures.
Also, there should be an option for people to register and and an option for the admin on whom to allow to comment. I mean, right now, commenting is open for everyone. I'm sure as Plogger becomes more and more popular, Plogger galleries will be prone to spam comments. If this happens too much, people may move from Plogger even though they like it.
The more important thing Plogger needs, is also Private Albums. Yet again, these albums must be visible to registered members. (So you'll need registration system here too.)
Other updates you could include in the long term: posting by email, hotlink protection, etc.