Turning the Hexblade "Patron" into modular powers

[Great Dragon]

Why not? It's as valid a play-style as straight fighter or straight caster.

Sure, but left as the Original Hexblade, means that none of the other Patrons are chosen except for flavor.

(With your "Curse spell" making it where the Player can get both their chosen Patron abilities plus Curse powers)

By that, there should also be non-multiclassing "Gish" full caster Sorcerer and Wizard options.

I suppose that Wizard's War Magic might qualify.... I haven't seen it in play, yet.

But, most players/DM don't have a problem with:
Eldritch Knight giving the Fighter/Wizard Gish option.

And Arcane Trickser giving Rogue/Wizard option.

*****

Maybe I'm just too much of a Grognard.

In a lot of Fantasy (and D&D in particular) there was a lot of "focus" on doing something:
Being a Fighter took lots of dedicated training.

Being a Mage was just as much dedicated training to truly master magic.

The Gish split their training between these two, and we're never as "powerful" as a PC that was "focused" on only one.
(For D&D, this meant that the Gish was not as "strong" in combat as a Fighter, or as "powerful" (spell level) as a Mage of equal - total - levels.)

Even RAW 5e tries to support that, which is why so many DMs disliked the "original" Hexblade.

Except for spell choices:
Made all the other options unwanted.