Nice work ... completing your Walker set.
I still need several.
Picking another series, can be a hard choice. Many of the
classic sets will have several harder key/semi key coins.
The
Mercury dime set, is not a bad set. Depending on which
brand album you buy there is three main coins.
1916D, 1921 and 1921D. Seems like there is one brand of
album that needs a 1942/1 and 1942/1D.
Indian Head/Buffalo nickel has several expensive coins.
And most of the early coins are hard to locate in better
grades.
One set I have been working... slowly working, is
2 1/2 gold Indian set. It is a short set and only has
one hard date coin. It looks great in a Capital coin holder.
Or some will do the set in graded slabs.
The set, not counting proof coins, is 15 coins.
Each coin might run from $190 to $300, except
the 1911D which lists $565 in G4 to $4500 in AU.
I have 6 different dates right now .. but for me
it is a long term project.
The coins are about the size of a dime.
Here is a example.

