I imagine an easy tie-in would be for the new member to be trying to track down the children - and they come across each other mid-travel? Or maybe we start off captured (a failed attempt to tackle the hag solo), when the adventurers find us?

Are there any restrictions or guidelines for potential characters?

Some sample character ideas that come to mind (I have more fleshed-out write-ups if any are worth considering as a possible fit):

• Chaotic Good / Half-Orc Sorceror - Religious Charlatan
• LASH was always on the road growing up, learning to deal with humans (and their stereotypes) by one-upping their fabrications, making up false histories more obscure than their rumors, and even concocting foreign gods and traditions to explain himself away. He'd would have made a decent Bard one day, but instead faced an early doom - had not one of his lies seemingly came true. He now masquerades as a cleric (of some non-existent god), a natural born liar banking on his own exoticism for profit.
• ** Lawful Good / Gnome Paladin - Vengeful Inquisitor**
• KORMAC is a somber gnome. His race lives a long time - but nothing guarantees they will enjoy those years. Disfigured in a childhood accident, he buried his parents and sibling by his teens - and then buried most of his extended kin in the decade that followed. He grew to work the local smithy and was resigned to a bleak and seemingly cursed future - had things not gotten worse. Combine a necromancer (defiling his dead loved ones), a group of adventurers (clerics, paladins and fighters) and a lot of bludgeoning - and days later the smith left home to join the travelers and eventually became a Paladin. Now he travels intent to hammer righteousness back into the world - or hopefully die trying.
• ** Chaotic Neutral / Tiefling Warlock - Confidant to an Old God**
• MAKARIA's only family was a group of brigands for which she played 9-year-old information gatherer - and when the local sheriff arrested and hanged them all, she was on to the same role - spying on and betraying other bandits, smugglers and the like to that same sheriff. Few trust Tieflings except to expect the worst from them - but they could never really imagine the worst. Twenty years and four masters later, she had become a deceptively small-scale player serving in court beneath a group of retired adventurers - a veritable viper's nest of backstabbers, bribery and bedroom alliances beneath a bard, a wizard and other miscreants. It wasn't until the wizard began calling upon Fiends, that their new 'guests' made Makaria's skin crawl - and she set to trying to subvert him (rutting with him and then prying through his things). It was among thus among his old artifacts and long neglected wards where Makaria first heard her patron's call and came to serve a new master, more terrible than any fiend. Now she travels, a half-delirious confidant, emissary and fledgling warlock to an Old God.
  

Just a few possibilities. I can toss together a fuller sheet easy.