I've been a fan of the Start Screen, but have been frustrated by some of the limits for customization. Below are a few suggestions. If any of you have your own, please add to the list in this thread.
1. Vertically stackable groups.
Currently, a lot of space is wasted if you have groups with only a few tiles or you use small-sized tiles. Being able to stack them vertically where you want to would allow users the option of taking better advantage of space.
2. Staggerable tiles.
The Start Screen interface won't let you place tiles in any way where they don't align along a set grid. For instance, small tiles have to fit into the space of a medium tile and be placed in a 4×4 configuration, and wide tiles can't start after a single medium tile. There's no real reason for this, and it limits your ability to set up your tiles in a way that you think looks good.
3. Universal or group-specific tile background colors.
Some people like the look on Windows phones where all app tiles are a uniform color. It would be easy to provide this option, as well as an option to set a uniform color that would apply for a single group only, rather than universally across all tiles.
4. Dual or multi-tone customizable pattern backgrounds.
Rather than complex visual backgrounds, which are often very noisy and visually distract from the tiles, it would be nice to be able to set pattern backgrounds with customizable colors. These can be dual-tone, and automatically inherit the Background and Accent colors you've set for your Start Screen, or they can be multi-tone and either guess the appropriate extra colors from the two options you set, or allow you additional customization options. The text color on the Start Screen could easily adapt to be readable, since the system knows what colors are being used. These might be: geometric patterns, abstract patterns like traditional Japanese fabric patterns, damask patterns, and so on.