Hmm, I'm not an insider but here's what I think:
Being able to assign "custom accent colors to all pinned app tiles" with the ability to control the transparency of the tiles is neat; in fact, if I'm not mistaken, I think I've seen leaks or screenshots of this feature in one of the technical builds several months ago - but if I also recall correctly, such a feature has never been confirmed with certainty. But to be accurate, if you instead meant "a custom accent color for all of the pinned app tiles" meaning just one accent color for all, then it'll be nice - but if you meant being able to set a different color for each tile, then that may be contrary to theming. How about being able to theme apps by type instead, e.g. games will be red, office apps will be green, chat apps will be yellow, etc.? This could either add noise or customization, depending on the taste of the user.
For the gaps between tiles, not sure why there's a need to make it easy to navigate between tiles in the start screen... what's there to navigate to? For me, having big gaps makes the interface less sleek or tight. It doesn't maximize the space; instead of displaying information, the space occupied by the gaps would be "wasted".