How about...
Everyone begins as a general adventurer. Balanced hp/mp, weapon and spell ability. No special class abilities of course. (Barbarians might be an exception to this? Otherwise they could learn some spells, then become barbarians and that wouldn't make sense.)
You can join a particular guild whenever you wish (or perhaps only post level 5 perhaps, by completing the appropriate quest). This will change hp/mp per level, and rate of development of weapon and spell skills. Also required to learn the skills of each class (circling and so on)
Assassins and thieves can covertly join their respective guilds yet still appear as "adventurers". I wouldn't let them masquerade as any other specialised class, the rigorous training required wouldn't allow them sufficient time to properly pursue their hidden career simultaneously.
The way I see this is that adventurers would be good all round characters, and playable in their own right. However, specialisation would mean faster and greater development in particular areas depending on which guild was chosen. People might have suspicions about those who remain uncommitted, but there wouldn't be any proof...
I'm kind of thinking that people need some sort of reputation that could be looked up. Kind of a log of achievements over time. So for example a fighter's reputation might be "Joined the fighter's guild age 18, having proved his strength in battle according to the demands of the sword master", "Advanced to the rank of myrmidon after overcoming a great foe". This would represent what is generally known about a character and could represent an alternative to displaying class and level. What if it was available in taverns, representing gossip? Also available in areas like the academy in Coral, for the appropriate class. For those who have clearly chosen a particular path, this information should be readily available. Could also include achievements like quests and titles.
Thieves wouldn't have so much in their reputations as they aren't officially joining their guilds (perhaps a special reputation only visible to other guild members detailing secret guild achievements). Perhaps though as they acquire experience through thiefly acts, they could acquire shady comments in their reputation (1000 exp through pocket picking = "rumoured to make a not entirely honest living"). Assassins something similar, based on number of contracts fulfilled? This would be unavoidable, as the character's activities are unlikely to remain totally secret from the rest of the population.
Another idea - if a particular nefarious skill is carried out in front of other player characters, this speeds up the rate at which the reputation becomes tarnished. Get spotted attempting to pick a pocket a few times? Reputation includes "lightfingered"
For those who wanted to play the kind of "solo" thief described by Ghandi, they would have to work hard to protect their reputation by keeping their activities unseen. Thieves who didn't mind being known for what they were could happily acquire checkered reputations, but then provide a ready point of contact for those who are in need of thiefly services. Lockpick for hire, kind of thing.
So if you happened to come across a lightly-armoured "adventurer" in the lands who seems to have been around for a while, it might be worth a trip to the tavern to buy a few drinks and see what's known about this person. Of course, they'll deny everything...
Lot of work to do for the DMs, even if by some miracle everyone was in total agreement as to how brilliant my ideas are! I'd love to hear your comments though.