There's a difference between "evil" and "you want these overpowered legendary pokemon you're gonna work for them." Nothing good is handed to us for free in Reborn; Beldum is the closest we've gotten and even then finding it is difficult enough if you don't already know how, much less catching it with that abysmal catch rate.
I'm assuming you got the types through matching the colors of the gemstones?
I like this a lot because I never considered the colors on the Reborn symbol to have significance outside of the keys; unfortunately, Charlotte is confirmed as only a gym leader, so unless Cal makes a surprise appearance later on (it's possible) I don't know if it would be fire. And Amaria is the only water leader we've ever had, so I can't say for sure that she'd be an elite four... Personally, just based on what we've seen ingame I'm thinking the Elite Four will look something like this, in no particular order: El (as much as i hate him considering we've never battled him in a full 6v6, past or present and he implied he can do better than a level 80 Ditto Arceus)
Laura (basically confirmed) Bennett (given that he intends to go off on a training adventure and El's "connections"; that and his team still seemed a little incomplete as of our last battle with him) and MAYBE Anna (considering she cut off Noel when you first met her, right before he elaborated on something involving her while Charlotte was talking about Laura: "She's also one of Reborn's Elite Four. Awesome kind of runs in our family. No big deal. NOEL: If you're going to say that, you may as well mention how Anna-" ANNA: Yeah! Laura's so cool! And Charlotte and Noel are also Gym Leaders~")
But, again, I don't know any more than anyone else here does as far as Ame's plans for the game goes.
Elaborating on the possibility of the keys representing the elite four, though, we can consider what Solaris told us under the grand stairway:
"That is Reborn. The city of insolence. It was built on four key powers. You can see it in the city symbols.
Observe! Ruby, the seal of pain. Sapphire, the seal of love. Emerald, the seal of faith. And Amethyst, the seal of the beyond." Perhaps if this is the case, we can try to associate those with characters? Given that Heather was (or, Corey was) in possession of Ruby, and the both of them pretty obviously fitting of "Pain"... Amaria's presently in possession of Sapphire (but do remember it was originally Laura's)- this is one thing I know that others possibly don't but let's just say that's fitting as well. Emerald having belonged to Luna is a little ironic, but in that sense (call it anti-faith, or maybe faith in the darkness instead of the light), still fitting- also considering El stole it from her, that makes it unironically fitting. Amethyst belonging to Anna and representing the beyond also kind of makes sense given she implies she has some kind of connection to Shade, and can see things the rest of the cast can't. Given this, Bennett doesn't quite fit in; but knowing Ciel exists as a potential Flying badge...giver, and we won't be seeing Heather for a while, that could fit her in the Elite Four as well. I know from experience Heather's no pushover ;-; Granted, these types don't match the colors much, but the characteristics do, I think.