Six suggestions...

I would like to see the following
(1) See thumbnails of files to split, without having to open preview page;
(2) split according to odd or even pages;
(3) Merge pages into middle of file, not just at beginning or end;
(4) Drag pdf file straight to desktop icon to open program;
(5) Bookmark pages in preview mode and to allow for splitting at bookmark later;
(6) Allow for file rotation before merging.