Are you looking to work in a specific weight of yarn? Two I have in my library that are simple but elegantly gorgeous are Westport and Where the Sidewalk Ends. Probably not going to be much of a challenge other than the size. They’re both super bulky, but I plan to do them in worsted when the time comes.
Bounce might give you some nice challenges with the lace stitches. You don’t have to do the color changes if you don’t want. Tin Can Knits patterns are great for beginner or new to stuff because she explains a lot in them and has good tutorials to go with them. The pattern is for stroller size, but she points out that you can increase the size by repeats of 12 stitches.
Which takes me to the question of how comfortable you are with math. Any afghan can be made wider than the pattern calls for as long as you can work out the stitch counts and longer by increasing panel length or adding panels. (And I started to explain the entire math thing but it got too long and boring. )
Ooo - found this one - Bird Song. The branches are done with applied i-cord, it seems, which might be an interesting challenge.
(I’m fascinated now by how few knit non-seamed blankets are like the one you made with different panels of stitch types. I might just have to design one!