Hi Chris
Before I begin: this is undoubtedly a moral dilemma for me - whether or not to help someone who is intending to bring more One Direction into the world.
With that said (partly in jest!) one factor that I think is missing from your list is retention - how many downloads vs how many installs. The better this ratio is, the better the play store algorithm will deem the app and thus it will promote it in the rankings. There's not much you can do to control this, however, other than making a quality app of course.
Freshness is also a factor - newer apps will get some preference and I imagine in a 1D category, newer apps might pop up virtually every day. So this could be impacting you.
Other than that, your standard SEO ranking factors I imagine would have some effect. On-page optimisation and inbound links passing PageRank will more than likely boost the ranking - but how much I'm afraid I can't say.
Everything else, ratings/downloads/reviews etc - you have covered. All I would suggest is maybe to look at some analytics tools. App Annie comes with a number of tools that provide you with the metrics for the top ranking apps by category. What I'd suggest is to look at the metrics, look for any patterns, and then throw in traditional SEO reports such as the on-page optimisation tool in Moz Pro/Analytics and any inbound links via OpenSiteExplorer.
Hope this helps. I hope you find that "One Thing" to help you rank higher.