So, a thought popped into my head. Was Greil's death against the Black Knight planned?
Why exactly the BK wanted to kill Greil wasn't really explained in Radiant Dawn. The explanation given was that he wanted to test his strength against Greil, his former teacher. But this doesn't really make sense, for two reasons: firstly, Zelgius trained under Greil. So surely he must have fought him countless times? Even if it were in training. And secondly, all he wanted was to test his strength, so why would he need to kill Greil for this? He could just have easily asked Greil for a friendly match, like Mia does with Ike. Especially considering he idolised him, doesn't make sense.
The point has been brought up many times that technically Titania should have taken command of the Greil mercenaries after Greil's death, as she's second in command. And Greil accepting the challenge from the Black Knight, knowing the consequences if he lost, right before the war breaks out, and leaving his 17 year old son in charge of a band of mercenaries who he has no idea to lead? Seems odd. Not to mention, he started preparing Ike not long before the fateful duel. Now, Greil had reason not to want to be around during the war with Daein, considering he would have been recognised as a former rider of Daein. Having Ike command the mercenaries would have made sense, as he wouldn't have been recognised. Also, Greil would have wanted to prepare Ike.
Rewatching the cutscene, the fight looks very forced, almost like Greil knows he'll lose. And considering he knows Zelgius, he could have had the whole thing planned out, preparing Ike just in case anything happened with the medallion. Greil ensured Ike constantly has a goal to work towards and a reason to keep training.
In Radiant Dawn we learn that Sephiran had nothing to do with Greil's death, so he wouldn't know anything about any other plans. And Zelgius didn't really have much reason to tell Ike, he'd got over it by that point and it would just overcomplicate things.
Apologies if I didn't explain it too well. But that's my theory. Thoughts?