Hi there - i'm not sure if center aligned text is officially supported by markdown - we could add it to the steempower editor but then it wouldn't show up here on steemit.

As for emojis and extra blank lines - the first issue is one i'm adding to the list, the extra blank one is "simple" but not a widely requested feature. You can do it by typing   on each blank line:
 
 
 
Or you can type <br/> on each blank line:






like this.

If more people request it I can look into adding a button to the editor of course.
Hope this is useful to you.