Wow, all the pictures posted are lovely! Now I feel the urge to make another sampler quilt. :D

Enjoy! I actually made nearly all the blocks and the ones in this picture are the ones I knew I wouldn't be washing!! So the other blocks were sent to my Canadian cousin for her group to make into quilts for the needy.

My first quilt class (1976-7) had us make a block each week, increasing in difficulty and using a variety of skills. My original group of fabrics was too large and I ended up setting aside all the blocks with a couple of fabrics that didn't look right with the rest. Finished quilt has 20 blocks, 8 fabrics in the blocks, including a light and a dark solid, with sashing between blocks.
I would recommend 8 or fewer fabrics, making sure any combination of them looks good together. I like your idea of having the sashing match the background of the blocks, but not all pieced blocks have a "background".
(Six blocks that got pulled out ended up in a quilted jacket.)

I did a sampler black and white with yellow. I took each finished block squared it off nicely, then did a square in a square with each block. I used the background white and a gray for that process and set them on point. Came out nice

Here's a great book: https://www.amazon.com/Anniversary-S.../dp/1617454559 It has several different layouts for 10 blocks up to 40 blocks. There are different sashings, applique borders or centerpieces, on-point or straight. I'm using it for inspiration for the sampler I'm making.

I never met a sampler that I didn't like. But for a worrisome designer, I would suggest you follow quiltedsunshine's advise and follow a pattern. Go to amazon.com and search "sampler quilt patterns" and 30 pages of a wide selection of patterns shows up. Pick one that appeals to you, select similar fabrics and/or buy a fat qtr bundle of a collection that you love and jump in. Doing it will prove to yourself that a mish mash of fabrics, patterns, and layouts really does come together nicely and the only mistake you can make is over thinking it. Another cautious approach is to join a BOW with a group making a sampler, and audition fabrics with friends one block at a time getting their feedback, input, and recommendations. Best of luck, we are pulling for you!

Also, look into quilt in a day, Eleanor burns......many of her books/videos on YouTube are sampler type layout.