I've been having this idea for ages. It seems that the spells for the warlock's otherworldly patrons are overly generic.

Example: If a warlock character wanted Orcus as their patron would obviously classify that as the Fiend Patron. But the fiend patron's spells leaves them are stuck with fire-themed spells like burning hands and fireball, which capture none of Orcus' aspects, unlike animate dead and ray of enfeeblement.

Another Example: A warlock who wants Dendar the Night Serpent as their patron would probably pick the Great Old One since Dendar is mentioned as an example in the PHB. But then that player gains no snake-themed spells or spells related to fear (except for fear), instead focusing on mind-themed spells.

Final Example: A warlock who wanted to have a trickster-like fey for their patron (who would like to polymorph someone into a toad), wouldn't be able to polymorph someone INTO a toad.

Therefore, I'd like to see more variety in the spells for otherworldly patrons.

Oddly enough, your example with Orcus made an appearance in Unearthed Arcana when it mentioned infernal and demonic cults. Each devil and demon listed had a small list of spells that their servants can have in place of other spells associated with them (such that a cult fanatic, who usually draws spells from the cleric list, could indeed cast Ray of Enfeeblement). I doubt this stuff will ever go into an official book, seeing as how many possible patrons there are, but as long as you aren't in Adventure League, you could probably talk to your DM. If you're playing a Fiend Pact (such as Orcus), show your DM the UA for fiendish options and see if they'll let you use the spells from your specific patron. Keep in mind that you probably wouldn't know them automatically, rather, they would be options for spells known. Your DM isn't obligated to let you do it, but for the Fiend pact, pointing to Unearthed Arcana might make your argument a little more convincing. Of course, my current DM is a huge fan of homebrewing, so there's that.

https://media.wizards.com/2017/dnd/downloads/UA_FiendishOptions.pdf

I Think WoTC did talk about something in that regard and mentioned it was too restrained and the testers didn't like it. It might have been the invocations but i think both have the same concept. To add to it, the primeval campaign for 5e did have the 14th level spell changed depending on you're old one (Cthulhu mythos only)

Quote from Arlong >>

I Think WoTC did talk about something in that regard and mentioned it was too restrained and the testers didn't like it. It might have been the invocations but i think both have the same concept. To add to it, the primeval campaign for 5e did have the 14th level spell changed depending on you're old one (Cthulhu mythos only)

Quote from Windrunner >>

Quote from Arlong >>

I Think WoTC did talk about something in that regard and mentioned it was too restrained and the testers didn't like it. It might have been the invocations but i think both have the same concept. To add to it, the primeval campaign for 5e did have the 14th level spell changed depending on you're old one (Cthulhu mythos only)

 Correct, it was patron specific Eldritch Invocations that were released in UA, but dropped from XGE due to the reasons you listed.

A lot of this comes up to DM's discretion, as always. I think a bigger or overlapping pool of patron-specific spells doesn't hurt anyone, and makes things both flavourful and makes the Warlock more versatile. After all, you gotta use those few spells per day wisely!