Whenever you add something or change something from the original pattern...you are modyfing it and I think thats when you are designing a quilt....
You may add extra sashing to blocks or take some sashing away or distribute the blocks in another way.....
Of course you may also invent your own blocks...(use a square paper sheet with some colors and play with it.....
You can always add some your choice borders to it
When I first started to quilt I didn't even know anything about blocks.....LOL the quilt on my avarat was done like this....
I don't recomend it since you need to "brake your head harder" on every corner of the way....LOL
Good luck.....