What is missing:

-system should remember tiles position on exit (even when active tiles are mixed with empty ones) and should not automatically rearrange tiles when one is dragged to new empty place (former place of relocated tile here should be replaced with [+] empty one; tile while dragged can look like dotted line contour, OR tiles can be dragged as they are with just empty space left on their former place (further click on such empty space should add new tile))
-search engine change (otherwise that may look as a kind of pay-per-click referral stuff) and option to hide search form section since it takes pretty much space
-'colour tiles' potentially is a good option, but without possibility to set own colors it does just a half of job: with manual color set it would allow user to visually separate tiles by groups
-when last available tile is used, new [+] empty tile may appear (just optional may be)

Everything else IMO is good as it should be. Neat, lighweight, well-balanced add-on comparing to others. Just few key things to improve.

Will update comments/rating when anything from above is changed.