It really depends on the definition of "versatile."
If what you mean is "the primary profession offers the ability to fill different party roles," I would probably go with Ritualist or Paragon. Both have a lot of defensive capabilities, both in terms of "protection" and "healing;" both have DPS capabilities - Paragons with Spear Mastery skills, Ritualists with Spirit Strength; and while the Ritualist has the offensive edge with Channeling magic that mimics "nuking," the Paragon certainly wins defensively with "They're on Fire!" and "There's Nothing to Fear!"
On the other hand, if you define "versatile" as "capable of running skills from any secondary profession and 'mimicking' any class," the Elementalist and the Necromancer win hands-down. Both of these professions have a primary attribute that provides excellent energy management, and since the only way to be as good at a secondary profession (without access to runes and that profession's primary attribute) is to work harder and cast more, energy management is what really allows a character to function well in such a situation. I may be biased (because of my main character choice), but I believe the Elementalist has an edge here: the Necromancer's energy management requires things to die rather frequently, and while things most often do in PvE, I prefer the Elementalist's "guaranteed" energy pool and the superb energy recovery of Ether Prodigy.
(On a side note, an Elementalist can tank rather well if equipped with the right Earth Magic enchantments, so it could also perform better as a "frontline" character than a Necromancer
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As far as your new character goes, I'd advise you pick up something you haven't played in terms of the party role. If you've played your Assassin as the "tank" - lead the heroes/henchies into melee, provide constant damage, etc. - consider a caster instead, or perhaps a ranger, to see the battlefield from a different perspective. Unless you've got all skills unlocked (which I doubt), I'd also recommend you pick both a primary and a secondary profession that are new to you. That way, you will be able to unlock more skills as you go through the game, which will benefit your heroes even if you end up dissatisfied with the new character and relegate it to storage
