Fillers:
http://leahday.com/pages/quilting-design-gallery
Motifs/Borders:
https://www.youtube.com/watch?v=AnAriXmGjPA
If you do pebbles, just make sure they are at least 1" in diameter (preferably bigger) or the thread build up will cause the quilt to become stiff & heavy (ask me how I know).
On my first quilt (44x56" crib) I did a basic curvy 1.25" meander in the center with a leafy vine in the border and a triple leaf in each corner. I picked a white Aurifil 50wt/2ply thread so it would blend in with the background. In the border, I used a variegated blue Aurifil 50wt/2ply thread. I had become pretty decent at FMQ by the time I reached the borders & wish I'd picked a thread that could have shown off my skills. I now use Aurifil 40wt for most quilting & occasionally even add in a 28wt thread if I really want to show off.
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My biggest piece of advice is to
keep it simple! There are plenty of us that could post lovely pictures of fancy feathered wreathes & scrolling vines & whatnot, but I suggest you work up to doing those things. I remember an online video I watched that said if you can quilt the lowercase letters c, s, "cursive e", o, v & upper/lowercase l, you can pretty much FMQ any design because most designs are just a combination of those. For example, feathers are a combination of a c & a v. Swirls are a large c that comes to an inner point (v) and then returns back as a smaller c. Unless you're planning on making competition quilts, FMQ really isn't that difficult. About 8 hours of practice & you should be able to stitch out a number of different beginner designs. The more you practice, the easier it gets. Also,
I always mark my designs on quilt tops before I layer my sandwich. It gives me a rough idea of where I want my stitches to land so I can focus entirely on stitch formation rather than dividing my attention.