Improve relations with Morocco from Day 1 for FBR and go for exploration ideas... you might not want to break out of PU that early on. Being a junior partner or a vassal isn't that bad - if you're OK with not being able to wage wars on your own for a while.
* As Perm (vassal under Muscovy) I colonized my way into Siberia before DoW'ing Muscovy for independence. Not a really serious challenge, I should say.
* As Sukhothai (junior partner under Ayutthaya) I simply waited and did nothing in the mean time, confident that they'll lose the union one day. It's easier when a union naturally dissolves, because you can ally the nations you've been improving relations on almost right off the bat, unlike wating an independence war where you need to win before you could ally people. I conquered Arakan, moved the capital there, abandoned Sukhothai and GTFO.
* As Khmer (vassal under Ayutthaya), I merc'ed it up and DoW'd Ayutthaya, taking how many loans I know not.
* As Norway (junior partner under Denmark), I almost felt sad that the union broke. I was colonizing North America, and I could finally DoW and annex the natives.. but at the same time I lost the protection from Denmark and her allies.
I wouldn't say Naples is really hopeless since it still has a port and you can at least colonize. I mean... Conquest of Italy can wait, and you might as well be doing things while you're waiting anyways. In many ways I assume it'd be easier than, say, the Anatolian OPMs like Dulkadir or Ramazan. I'm still trying to figure out how to make France give me my core Labourd in the HYW as Armagnac... DoW'ing France for independence will be so epic.
Being under personal union is still easier than vassalage. There's a chance it dissolves, unlike vassalage which can only happen if somebody forces release.