Besides the Idol-Head of Diabolu, there were several other noteworthy plotlines left hanging at the end of 1964. 
One began in Detective Comics #334 where the Caped Crusader met “The Man Who Stole From Batman.”
In this Gardner Fox/Carmine Infantino story, the super-leaping Grasshopper outmaneuvers Batman — lures him up to a roof, then jumps down — and tops that by somehow hotwiring the Batmobile and driving off. He follows that up with the theft of a batarang, kidnapping Robin and finally preparing to steal the Darknight Detective’s life away.
The Grasshopper’s secret edge is that he’s actually twin contortionists. When Batman ran up to the roof, one twin hid in a “no point in looking in there” space while the other drove off with the Batmobile. The other secret is that the twins are just catspaws for … the Outsider.
Who is he? We don’t know. At the end of the story, a call on the Batmobile’s car-phone (back in the day, car-phones were way sophisticated) turns out to be a pre-set message from this mysterious villain. As you can see below, it’s not “Congratulations on besting me!”
While I didn’t come in on the Outsider saga until much later, this opening still impressed me when I finally read it. Sure, the Green Goblin hid his secret identity but a mastermind who doesn’t even appear in the story? And won’t simply pop up in the next issue so Batman can wrap things up? That was different. Even knowing this arc doesn’t stick the landing (I’ll get to that eventually) it’s a cool way to start.
Unlike the Outsider’s plotline, the Iron Man story arc (by Don Heck and Stan Lee) that started in Tales of Suspense #59 ran steadily, issue to issue. In the first installment, a lack of power in his chestplate affects Tony’s heart so that he almost blacks out. Providing a power boost to the chestplate fixes things but also leaves him stuck: if he takes off the armor the power will fluctuate and he might black out again, or worse.
I’m not sure this makes sense — why not just plug the chestplate into the wall so the power stays constant? — but it works because the core of the story is Tony’s own fear. The possibility that his heart’s going to give out leaves him terrified, which leads to great drama. Iron Man has to tell Pepper and Happy that oh, Mr. Stark left town, I’m in charge now. They’re not buying it, suspecting that Tony’s trusted bodyguard is up to no good. Sure enough, they find him in the next issue looting money from Tony’s safe, though Iron Man explains it away (if Stark hadn’t authorized the withdrawal, how would he know the combination?). I’ve read the ending and I don’t believe this arc sticks the landing either. However it is one hell of a gripping set-up.
Justice League of America #32 was a much simpler cliffhanger. In “Attack of the Star-Bolt Warrior,” (Gardner Fox, 
Mike Sekowsky) the JLA suffer mysterious power fluctuations, track them to the source and become captives of Brain Storm. Said villain blames Green Lantern for murdering his brother, a petty hood (spoiler: not what happened) and wants the Leaguers to function as a jury when he pronounces sentence. It’s a fun story, mostly because Brain Storm’s clueless about using his stellar energy helmet. He doesn’t plan or think strategically, he simply does whatever pops into his head when the energy he’s channelling triggers a brain storm.
At the end of the story, with the JLA firmly having the upper hand, Brain Storm’s inspiration is to vanish before they take his helmet. He disappears and the League can’t find him. However they know he’s out there — somewhere — and they haven’t heard the last of him. Having villains escape wasn’t as revolutionary as I thought when I first read this; Magneto and the Green Goblin did so routinely and even at DC, Dr. Tyme (yes, the guy from Doom Patrol‘s second season) pulled it off around this period. Still, there was something so uncanny about Brain Storm’s departure that even rereading now, it feels less like a retreat and more like a win for the villain.
I’ll be back with more from my Silver Age rereading later this week.
#SFWAPro.
RE: “(…) a lack of power in his chestplate affects Tony’s heart so that he almost blacks out.”
It seems like Iron Man’s armor flashing a low battery warning was a frequent storytelling crutch used by Lee, Heck et al. throughout the 1960s. I touched on this in a post here a few years ago in which this was the key plot-point of an extended run of IM stories in Tales of Suspense and then his new solo title.
It was indeed used as a crutch, though Lee/Heck could also get some good drama out of it (as in this arc here). Tony’s heart made it more effective for that than, say, Spidey running out of web fluid.
Archie Goodwin, who took over the title (he did those issues you were posting about at the link) eventually gave Tony an artificial heart to get away from that, though not completely (the heart was experimental, it gave out if he exerted himself …). In the long run, it was less of a game-changer than him introducing Howard Stark to the book.
Huh. Reading my earlier article about Howard Stark I saw a note that Tony turns out to be adopted, went to look it up. While that’s apparently still canon, I discovered a plotline in which Howard turns out to be the agent of Mephisto, having sold Tony’s soul to the devil …
Which turned out to be a Howard Stark from an alternate reality (no surprise, that was obviously never going to fly) who has now become the Iron Inquisitor …
And it turns out the Mephisto is one of those puppet master villains who turned out to be behind everything — he inspired Apocalypse and Thanos! He convinced the Hulk to turn against humanity! — which is the kind of big reveal I always hate.
Jason Aaron isn’t one of those writers I absolutely avoid but outside of Ghost Rider I don’t like his superhero work much.