At maximum, all we need is three of each class of hero, thus allowing you to fill to your limit. Let us assume, for the moment, that the GWEN's 10 heroes correspond to the 10 professions.
Warrior- Koss, Goren, Smexy Norn lady.
Ranger- Margrid, Jin, Insert-GWEN-hero-here.
Necro- Olias, Whispers, GWEN.
Elementalist- Sousuke, Zhed, GWEN.
Mesmer- Norgu, GWEN. Attention required.
Monk- Dunkoro, Tahlkora, GWEN.
Ritualist- Razah, GWEN. One here.
Assassin- Zenmai, GWEN. Yet another here.
Paragon- Morgahn, GWEN. Insert requirement here.
Dervish- Melonni, GWEN. One needed here.
So, making the vast, purely-conjecture-based assumption that one of each class will be added(sketchy as that may be), we need a Mesmer, Ritualist, Assassin, Paragon, and Dervish.
Kurzick- Ritualist. They've got the traditions, the history, the song, etc. Perhaps an exalted member of the power structure on one hand, maybe a rebel against customs and ceremony. With the dredge nearby, the Kurzick should certainly be interested in the tunnel system and What Lies Beneath.
Luxon- Assassin. Swift and decisive, like a striking serpent. Massive shattering across a sea of jade tends to attract notice. As a result, it seems a good idea to send in an expert scout, a stealthy survivalist to investigate the circumstances.
Paragon and Dervish from wherever in Nightfall. Anything from a simple Kournan from a recovering empire, a Vabbian dedicated to cause of preserving the world, or another corsair, although those have been covered. If we want unique, a member of Palawa's undead horde, a turned Margonite like the Lost, or even one of the Djinn would be nifty. (Personally, I'd love to see an undead cavalier as a companion, as well as a sapphire djinn like Dehjah. Interesting history, plenty of personality possibilities, and nifty graphic options abound.)
That leaves the Mesmer, which would easily be Tyrian. The most likely location for such an individual would be the Shining Blade, who probably don't have much to do after the upheaval of the White Mantle. A crystal desert ghost would be a little wierd, the Ring of Fire doesn't seem to have a large population, and Ascalon needs all the people it can get to rebuild. So, one of the Blades, preferably of the attractive variety (no offense, Norgu, but you aren't exactly easy on the eyes.), who serves as another link to the original campaign.
The one obvious problem is that GWEN is still a ways away. So, how do they fit in, when we, being the impatient players we are, want them now?
Remember when the Tomb of the Primeval Kings was ensnared by Dark Tentacles of the Shadowy Shade of the Umbra? Great times, and fantastic prelude to that wonderful fall into night.
So, let's do the same sort of thing. Kurzicks investigate the Dredge, who launch an all-out blitz in response to a disturbance beneath the ground. The outcasts, who congregate around Kanaxai's den, suddenly frenzy around the Archipelagos. The Djinn are concerned about recent rumblings, and the shifting sands of the desolation are picking up tremors. Even the druids of Maguuma are concerned, transcendent as they may be.
As a result, each campaign gets a few minor quests as a prelude to GWEN, with new heroes, a sneak-peek of lore, and a growing anticipation of the upcoming game.
Thoughts?