I just purchased a new beechwood spice rack and it has a lovely 48 containers.
I have about eight that are empty. I have been cooking for over 30 years so without giving you a long list, be aware that I have "the basics"...and then some. What I'm looking for is a dry ingredient I can add to my spice collection that is a bit off the beaten path, but that you use often.
Expanding the "spice" category to include spice blends, I'd add za'atar, a middle eastern spice blend comprising sumac, thyme, sesame, salt. I like it as a flatbread topping and on vegetables (e.g. sauteed zucchini).
+1 for za'atar.
and speaking of blends...vadouvan is another good one if you cook a lot of Indian dishes, and shichimi togarashi for Japanese recipes.
as far as single spices go, don't forget the smoked paprika!
What about garam masala ? Pretty popular spice blend. I can also see almond powder and walnut powder may be useful. Many love Sichuan pepper powder.