Prince Charon said:
A villain who knows Peter's secret identity (possibly Doc Oc, but I was thinking someone less competent) kidnaps all of Spider-Man's previous girlfriends (possibly even using a stolen time machine to gather anyone who's dead or otherwise inaccessible, but that way lies massive angst), in an attempt to blackmail Spidey into working for him. Unfortunately, Spider-Man is off-planet with the Avengers. Unfortunately for the villain, Spidey's girlfriends are generally not helpless or stupid.
This could be a fun one-shot or short bit. Actually, it gets further complicated if they end up kidnapping people who were only thought to be dating Spidey/Peter. Please note I'm cherry picking the girlfriends I'm most familiar with/would cause the most property damage regardless of the continuity.
So MJ Watson(-Parker, dammit) is definitely on the list. She's proven herself able to keep levelheaded in a crisis situation and is capable of handling herself. For extra fun, perhaps she has the web-shooter bracelets Pete gave her way back when.
Felicia Hardy aka Black Cat. This is already looking worrisome for our kidnapper.
Kitty Pride. X-man known as Shadowcat. Mutant with the ability to pass through things. Going by the Ultimate-verse continuity, dated Pete in High School.
Carlie Cooper. NYPD CSU officer. So now the cops are after whoever this is, and honestly, if we go by CSI: NY as a basis and the comics she was in, she's quite competent and capable of taking care of herself. Even more trouble for our kidnapper.
Sarah Rushman, who is in actuality the S.H.I.E.L.D.-brainwashed mutant Marrow. Making this really worryingly awkward for our kidnapper given Marrow in general has a fairly relaxed attitude towards having a body count or maimings in general.
Liz Allan. Dated briefly and depending on the continuity is either married (or the ex of) Harry Osborne, aka Green Goblin 2. OR she's a mutant with fire powers. Either way, MORE pain for our would be kidnapper.
Then for the piece de resistance: Carol Danvers. Miss, now Captain Marvel. Usual suite of flying brick powers. Guaranteed to cause issues for any kidnapper.
And for extra fun, other women who have been friendly with Spidey but never dated him:
Jennifer Walters, the Sensational She-hulk. Known to snark off with Spidey during the brief occasions when they've interacted.
Jessica Drew, Spider Woman. Completely non-spidery type powers, but going to be a huge headache for kidnappers nonetheless.
Things REALLY don't look good for our kidnapper at this point.
Also, more fun when they start gossiping about Spidey. Which gets really awkward for him in the epilogue when he finally gets back to Earth, hears about what happened, swings off to the rescue, only to find all the women sitting around drinking coffees. They all look at him, trade mutual amused glances then everyone starts giggling.
EDIT:
Also, if one were to put stock in rumors, then someone's liable to add Human Torch and Iceman to the list. Not only are they more trouble, this is even more fun for the rumors.