Here are a few ideas to make Sia more attractive for renters and get more contracts filled.
It's been mentioned a few times, but persistence of files across clients is a real deal maker/breaker. This is in line with what people expect of cloud storage. The holy grail is storing files in a HD brain wallet. That would turn Sia into the most powerful cloud storage system ever, full stop. It will be an instant hit among crypto enthusiasts, journalists, etc...
Also, If I suffer a hard drive crash and lose my wallet files, I shouldn't also lose access to all my stored files, in fact, I kinda used Sia to prevent such a thing from happening in the first place. Having to back up a wallet file in addition to saving my files on Sia is counter intuitive. People will get confused, lose files and freak out if this is not sorted out.
Also on the front of avoiding catastrophic failures, extending the renewal system. If you could preload a given file to be saved for say 1 year and then don't have to worry about keeping the daemon open, that would be fantastic. If I want to use Sia as a long term archival database (and I actually do), I sort of want to stash my files and forget about it without having to remember to run daemons and port things over when I change computers or have a friend use the app if I go on an extended holiday. At the very least, an independent warning system (email?) should be put in place that reminds people of the cutoff date, though I'm not sure how you would do that. The varying price is a challenge to ensuring long term reliable contracts, but there is a need for long term hands free storage even just for the sake of convenience. Maybe an intermediary dapp/contract that you can over provision with siacoin and whose task it is to keep your contracts going as long as there are funds?
Using Sia as a folder in your OS, in a way that hooks into the file system in a proper low level way like a symbolic link or a remote drive or something. Also in line with expectations around this kind of service, and just necessary and mega useful for all kinds of applications.
Not having to upload the file once for every redundancy factor unit. If there is absolutely no way around this, perhaps a bridge system could be devised, where the files are transferred to a carrier service that is then charged to distribute it to the appropriate redundancy in exchange for a Sia fee. The difference between uploading 100mb and 500mb is already too much for many/most people.
It's true that splitting the renter and host apps is a good idea. Segment and target your markets in a more focused way. Kind of like how it makes sense to split Bitcoin wallet and miner apps the same way those communities can overlap but often differ.
In-client Shapeshift.io integration. That would make the wallet more coin-agnostic and friendlier for people with Bitcoin that might not be familiar with alts.
In the same vein, a commercial service running on Sia that lets people pay in btc or fiat. Such an offering could be an instant hit depending on how attractive you can make it. The prices could be adjusted in real time according to the offer, guaranteeing the absolute best deals. That would really help drive demand.