One reason I accumulated the number of completed/non-quilted tops that I have (around/over 12) is that I couldn't quilt them down to my satisfaction on my little old sewing machine. For the past couple of years I now have access to a long arm and I'm pretty happy with what I can do.
But I find that while I think just fine in terms of 2D and the block designs/quilt patterns, my eye/mind have not fully evolved to the 3D of adding a quilting design and while some of my tops I do know what I want to do, some of them I have no clue. It does not come easy to me even though I am thinking about it the entire time I'm piecing. I'll do a search on the quilt name and look at what other people do (mostly on pinterest) and get ideas of what I do and don't like.
I see at shows the work of people who get the concept and I am learning, but not quite there yet (if ever!). Another issue I have is I still think in the more traditional hand quilting types of designs (individual blocks) and not so much in continuous line concept. I find with my friends machine that a stop stitch and then baste to the next block works better for me than stopping/starting/cutting threads -- her machine will NOT travel the uncut threads from block to block without giving me issues.
While I can work with the smaller/tighter designs, I still prefer a puffier acrylic batt to the warm & natural type. I just prefer the weight/feel of it. There are times though when a tighter design with the warm & natural is the correct choice.
I have some vision issues and the long arm is not set up for doing pantographs (no back handlebars, no computer either). When I'm not doing free hand designs, I copy the designs onto parchment paper I buy at the dollar store (rolls are 1 foot x 25) so I can see them clearer. It copies well and holds up much better than tissue paper but is still quite easy to tear off. Of course, tearing off the paper is messy and takes a lot of time (also to copy the designs in the first place) but the final results are worth it to me.
Typically my quilts don't have a lot of "empty" space. One thing I've found is that while I can do quite intricate designs the work gets lost in the busy-ness of the fabric and or block design. Best for me is to keep that sort of thing into alternate blocks, for instance I have a top of small "Broken Dishes" (maybe 4" finished size??, they were made from leftover cutting scraps of another project) separated by plain blocks. I think I will be quilting small butterflies into the alternate blocks while leaving the pieced blocks alone, or maybe just one (or both) diagonal line through the dish blocks.