I think McGann would be good, if only to see how 10 & 11 react/treat him, knowing he was the one that used "the moment" that they still feel guilty about.
I realize this is not the point of the thread, but I'm not convinced this is the case. I know it's always been fan assumption that Eight was the one who killed off the Timelords and the Daleks. But given the events of "Journey's End," I think it was actually a brand new Nine in the throes of post-regeneration instability.
In "Journey's End," real-Ten specifically likens human-Ten to Nine right at the beginning of the show, when Rose met him, because he just set the Crucible to explode and destroy all the Daleks. He describes it as "born in fire, born in battle" to explain his more bloodthirsty nature. (In fact, to drive home the point, I think it would have been nice if they'd had human-Ten wearing Nine's leather coat rather than matching brown-or-blue suits.)
If Eight was the one to use the Moment, then he didn't do it because he was "born in battle" as Ten seems to be suggesting. Eight was old and experienced by that point, not young and crazed. So that leaves Nine.
My theory is that Eight fought the majority of the Time War as we have always assumed, but that in one of the biggest final battles Eight was "killed" and regenerated into Nine. And it was Nine, in a post-regeneration fugue, with chaos and death all around him, who used the Moment and killed all the Daleks and Timelords.
That also fits better with "The Parting of the Ways" IMO. If Eight was the one who killed everybody and then
regenerated, why would Nine regenerating again be closure for him, as PotW implies? If Eight had killed everybody, then Eight would be the one crippled with guilt, and regenerating into Nine would be the new slate. But Nine's behaviour doesn't fit with that.
Nine is the one who carries all the guilt (far more than Ten or Eleven). So it only makes sense that Nine is the one who took the actions he feels guilty about. He killed everyone almost the moment he was "born in battle," so he spends his life full of guilt, until he finally sacrifices himself to make up for it. That's closure. Nine sacrificing himself to make up for something Eight did doesn't make dramatic or emotional sense.
Anyway, that's what I think.
As to the actual topic of the thread, I would love Eight to appear in a multi-Doctor story. I know he's probably the least well known of all the Doctors, understandably, and thus the least of a draw for non-hardcore fans. But I think as a character he deserves to have at least one more on-screen adventure.
Ten is too recent (it feels like the same reason why Tom Baker didn't want to appear in "The Five Doctors" - he only left 10 minutes ago), plus he's too similar of a character to Eleven. Nine apparently doesn't want anything to do with the show anymore. Nobody knows who Eight is. All the rest are either old and fat, or dead. (Although I think Six would make a fantastic character contrast for Eleven.)
But if we must have Ten and Eleven together, maybe it could also involve River. I think there's some interesting dynamics to be had there with Ten teasing Eleven, and Eleven being hideously embarrassed to be pegged with "human squooshy stuff," while Ten is also morbidly curious to meet this woman again. It would also help to explain how come River instantly recognized Ten in the Library but didn't instantly know that Ten wouldn't recognize her - she'd already met him once.
Although if Eight were involved, that precludes River, or else Ten would have recognized her. And Nine just wouldn't give a toss about any of it. He'd just sigh and roll his eyes.
I think there was about one sentence of that which was on topic.