A lot of fighters only tend to want AP or AD. The only exception I can really think of is Jax, who in some situations wants both.
For Nasus in particular, you generally want to run Grasp or Spellbook as your keystone, and take either inspiration secondary with Grasp or precision secondary with Spellbook. Conqueror also reportedly works, but I have not verified that. Nasus is unlike other bruisers in that he doesn't need to build damage.
is a perfectly viable build on him (assuming, of course, that the enemy team justifies buying Thornmail and Adaptive).
His meta build generally has Trinity instead of Iceborn (better powerspike and you can generally get it on the same back you'd get Iceborn), and also tends to use Sterak's, but that build is also viable.
Nasus also doesn't really want AP. It's not entirely wasted on him if you get it because it's on something useful, but it's not something you should itemize (No. Not even in ARAM, unless your team has zero AP otherwise). AP Nasus is...well... Honestly terrible.
And you can very well run Sorcery on non-AP champions as well. I run it quite often on Ashe and Caitlyn for various reasons. Sorcery has Manaflow Band (useful on a lot of champions in a lot of situations, and most of the reason I run this tree), Transcendence (Tanks in particular tend to love this rune since a lot of tank items have CDR, and it's fairly easy to overcap in their case), and Scorch (strong on any kind of poke, not just AP) and Gathering Storm (General good-scaling rune). Don't think of the trees as being for a specific type of champion, think of it in terms of what the trees have.