Hehe
My thought for Shard is that the character is actually the crystal. It's a tiny fragment of a goddess that was shattered eons ago in a tiff with some horrible thing...customizable to campaign story!...and was scattered over plane, space and time. A chunk wound up hitting some poor bystander in the head and boom. Weird synthesis-kinda-possession scenario. Shard is almost definitely a Light Cleric.
Grandfather's story is less defined to me. I don't even think he really understands why he keeps coming back the way he does. He has some theories, but he doesn't spend a lot of time dwelling on it. Despite his cantankerousness and his being a drama king, he loves being alive...the bad with the good...so he has no -actual- complaints about his situation. Because of that same situation, he has a broader idea of what constitutes 'nature' as well, hence his involvement in the planes. He doesn't share the insular "protect our world from the interference of other worlds" mentality a lot of druids have...and regards such sentiments as hopeless naivete at best. That's why a master/apprentice thing would work for him in Planescape, conceptually. He'd probably be fairly unwilling to take on a pupil for the most part, especially before he recovers his powers...but finding an upstart druid with crazy ideas about the planes and nature, ostracized by his/her peers? Yeah, he'd want to nurture that.
Manticore would be a lot of comic relief at first, but I also like the potential to explore the brain-twistiness of Planescape with him as a vehicle. What's really going on with him? He doesn't disappear when he 'logs off,' but that only seems to happen when he sleeps. Then he 'dreams' of another life in another world. Is he being possessed? Is he what he claims? And more ominously, if he believes this, and others start to...will reality -become- what he claims? Or has it already?! He's the martial pole of my trifecta, being a fighter or ranger most likely. A hard hitting warrior full of badassitude. Though, you know I could almost see him going Bard too, with his 'inspiration' being represented by those pre-raid strategy talks where everyone is assigned their job. The irony being that despite his evident psychosis, he's actually pretty good at tactics in this 'reality.'
Macrisse's dirty secret is that he (in my headcanon) actually accidentally is responsible for his own situation, and his blustering about taking revenge is a smokescreen so he doesn't look like a total moron. It's also entirely possible he's not actually a dragon, and is making that up too...perhaps taking advantage of his sorcerous powers to give credence to the claim, or perhaps he's a bit delusional and has convinced himself that his story is the truth!
Omega's the only one here who doesn't revolve at least a little around questions of identity and relative reality. He knows exactly what he is, and he knows exactly what he's meant to do. He's just not entirely sure he wants to do it. So for now he's tooling around, collecting information on how things work here so that, if he decides to, he can wipe everything out later. He has deteriorated quite a bit during his long inactivation, and his previous reality was QUITE a bit smaller than the Great Wheel, so he has a lot of work ahead of him if he goes that way. But one thing that deteriorated too was whatever part of his mind that was locked unerringly on his function; he has more free will now, and the novelty is interesting to him. I'm not completely sure what he'd be, mechanically. Perhaps a fighter with the 'eldritch knight' path, with his spells representing weird, alien technological 'magic.' That feels pretty good to me.
I am pretty familiar with Planescape as a setting, and while I am not yet making specific plans for factions (I like to keep things fluid to start with), it's true that some of these ideas play rather exceptionally well for certain of them.
