I think the consensus is clear, that a half-caster version of the bard (and I agree with those saying the Warlock, also) is the way they should have gone. Rather than, yet another "Charisma-based full caster."
Paladin = "Divine" half-caster.
Ranger = "Divine[Nature]" half-caster.
"Arcane" half-caster = ??? Why wasn't one created? Why was arcane magic basically dealt automatically 4 [!] "full" caster classes and a couple of "1/3rd" casters subclasses. Why are/were there no 1/3rd divine/nature-based caster subclasses?
I fluctuate in how best to answer this...and the bard/warlock connection, mechanically, might even be interchangeable.
Warlock being an Intelligence-based Arcane half-caster. Essentially, the "Half-caster Fighter-Wizard" option (as Paladin is a half-caster Fighter-Cleric) with additional magical [but-not-"spells"] powers. As with the Paladin and Ranger, this gets them access to full armor (at least eventually) and weapons options. If you want to keep the "Invocations" as their signature "powers" that's fine. I never much cared for that particular term, but the idea of a separate mechanical magical power (a la the paladin's channeling/auras). A limited, but thematic, addition of bonus "sub-class spells." Armor, weapons, 5 Arcane spell levels, limited bonus spells (Patron based), special magical powers only warlocks can use (Invocations or whatever you want to call them).
"Patron" subclasses, then, give you the various types of "more demony" or "more feyish" or "more Cthulusque" (or whatever other patrons you come up with) flavors. The secondary "Pact" options...I suppose just stay as/is. A secondary class feature independent of your "subclass."
Bards, then, can be the Charisma-based Arcane half-caster. Essentially, the "Half-caster Fighter/Rogue-Sorcerer" option (as Ranger is a half-caster Fighter/Rogue-Druid) with additional magical [but not spells] powers. As with the Paladin and Ranger, this gets them access to full armor though I'd probably limit it, with the thief angle, to light and medium, no shields. Simple and a few martial weapons. And their special class magical powers/mechanic all their own, call it the Inspiration Dice that is used to magically influence others (as it does now), add in fascinate, or lore checks, spending it for a bonus spell/slot recovery, etc... Armor, weapons, access to 5 Arcane spell levels, special magical powers only bards can use, and -like the Ranger receives additional skill and damage bonuses (either Hunter preferred enemy/fighting style stuff or Beastmaster companion damage), instead of the Cleric/Paladin "bonus spells" system- give bards some extra skill abilities. I'd probably based around gathering, researching, extracting, etc... lore/information on any variety of topics and/or Rogue abilities, makybe sneak attack style damage bonus.
"College" subclasse, then, give you the various types of "more Fightery" or "more Wizardy" or "more Roguey" or "more Clericy/[or traditional] Druidy" flavors.
There's your half-caster "arcane" Bard.
