1.  For "blowing" leaves:  I would definitely add more but change the angle.  Right now they all point the same way and line up.  I would also change the size of some of the new ones.  Leaves in nature are not like cookie cutters.  Perhaps strew them down and to the right.

2.  I am not familiar with the paint and any problems it might cause, but I would quilt it in very elongated "sort of" oval shapes like the long whispy clouds that flit across the sky.  I would experiment with a smoky invisible thread or a variegated very light gray or blue tone.

3.  To add texture in the quilting, I would make a free motion bark on the trees using variegated threads.  I would also add some quilting to the grass area.  Again, variegated threads or even some specialty threads, but not too much variety.  The quilt will end up too busy. 

     Another consideration might be to add some leaves "floating" outside the main quilt into the border on the lower right corner.

If you use monofiliament, ( dark), a #60 or #70 Microtex Schmetz  needle can be used .  That would minimize the problem with needle holes from the freemotion quilting.

It just seems a bit heavy or ominous right now.  If you have the leaves falling at different angles it should look more spontaneous and natural.  Also, quilting in a leaf shape might perk it up with variegated brown thread.  Not sure of what to do for the clouds without a closer look.  Good luck---nice job.

It looks like your "wind" is blowing left to right. Try adding some thin quilted branches (like the lines seen when viewing a tree from afar) that appear to be bending with your "wind.

I agree with pinkmommaduck on texturizing the bark and quilting in some wispy clouds. You could even use some monofilament to quilt in some wind in rolling scrolls (think Van Gogh's "Starry Night").

Add some rain drops with clear beads. Beads come in so many shapes these days.

To make the leaves appear more natural, try giving them a fling in a big arm wave across the quilt. It sounds totally dorky but I've  had some success getting a random appearance this way. Start with a few--say five--and see how they land. If you happen to quilt in some leaf shapes, make them smallish simple shaped leaves like elm leaves.

Using a metalic or pearlescent thread add raindrops from left to right either as short dash stitches or small lazy-daisy embroidery stitches. 

I agree with some of the others that it needs more leaves with a more random look.  Sandy10 has a good idea for doing that.

Clouds are a prblem for me, too.  I have trouble getting them to look the way I want.  My best results so far are free motion with a zigzag or a very open scalloped satin stitch.  (Love those preprogramed stitches on my machine.)  Takes a little practice.  Try it out on a sample first.