EDIT: Yeh. I guess that's sort of the Lord Rule: You die you fail. But actually, I think Elincia would be considered a lord. A near-endgame lord, but a lord nontheless. And she's...TECHNICALLY...a pegasus knight. But yeah, in chapters where she's involved, if she OR Ike die, you lose. So. It works.
Sure, Elincia could easily count when you take FE 10 into account, she has a number of chapters with lord status there (2 chapters in part 2 and I think another 2 in part 4)
You're right she's not a lord. She is, however, par of what I call the lord-sub class archtype. Most games have a character (besides the jeigan) that has story importance, is sometimes needed for certain missions, etc.
Other Sub-lords (imo) are: Soren in Path of Radiance, Sothe (and Soren again) in Radiant Dawn, Caeda in 1, I guess I would calls Nils and ninian the sublords of Blazing sword (though only because there is no one else that fits the bill [3 lords ftw]), And I can' think if there was one in the sacred stones...
So I wouldn't count Caeda a likely addition...at all. Sub-lords are less important than lords, and even less than villains.
Gameplay wise I would rather compare her to the starter cavs, Lowen (Fe 7), Franz (FE 8) and Oscar (FE 9), she needs to be trained a bit but is easily one of the best units later on.
Sub-lords? Maybe, but if I had to define an archetype like that, it'd probably be characters that only retreat when they are defeated instead of dying. Soren definately fits that, unsure about FE10!Sothe at the moment, I think he's closer to both lord and Jeigan status though. In FE 7, the lord(s) that you didn't choose as main lord would fit the description, unsure about FE 8, didn't play that one in a while.
Personally, I think that the most likely additions are Hector, Ephraim and Sigurd (only when there is a FE 4 remake though). They are all well known and are, together with Ike, the strongest lords in FE history.
In case of a villain, Ashnard seems to be one of the most likely addition, alongside with the Black Knight (a bit similar to Ike I suppose, but on the other hand it shouldn't be too hard to come up with a few different moves). Neither FE 7, nor FE 8 really provide one as the final bosses in these games were a dragon and a demon. FE 10... maybe Lehran, but I really doubt it.
Overall, a villain seems to be unlikely when there are so many other characters to choose from imo.
EDIT: Ninja'd by two people... ingenious >_>
I doubt he is as well known as you make him out to be. Yes, he may be better recognized than say Sigurd considering Sacred Stones was released over seas. Still, from what I know there are many people who don't like Ephraim or Elena (though I personally did like Ephraim and wanted him to appear in Brawl).
I said he's considered one of the best, dunno that much about how well known he is in the US, but considering that it was the second game released there, a lot of people that picked up/liked FE 7, probably bought FE 8 as well.