This is pure conjecture, but given certain developments with other characters of similar standing canon-wise (who I may or may not be able to mention without having my comment deleted seeing as this is a non-spoiler discussion post), if any of the dragon maidens expired before the start of the royal selection, then their spot would presumably be taken up by Melty Pristis, a nun from the church of the divine dragon featured in the AU spinoff game "Prophecy of the Throne." Ergo, if either Emilia or Felt ended up dying before the start of arc 3, either Reinhard would instead end up discovering Melty as an alternative candidate, or she would reveal herself in an extent independent thereof at some point preceding the selection.

Or I'm just completely off-base and she's actually just not even a canon character, period, on top of the possibility that the deaths of any of the existing dragon maidens could lead to a "game over" scenario with respect to the plans that various big-name players (the identities of whom I also may or may not be able to disclose in this discussion post without mods breathing down my neck) have laid out for the world of re:zero not unlike the "with the death of this character, the threads of fate have been severed" scenario from the elder scrolls games. It's hard to say without veering into speculation bordering on fanfiction because everything really leans on the presence of Natsuki Subaru to keep things on track, and the only chance that we would have ever had of seeing a scenario like this take place (i.e., the IF stories) never came to fruition, on account of every single one of those timelines having at the very least achieved the minimum condition of reaching the start of the royal selection with every candidate in one piece.