I do think Whitefang was hinting at Idoneth vs Fyreslayers.
1) Ages ago I mentioned the idea of a Nurgle vs Idoneth box and he said "choose your opponent.
2) I specifically asked him for a reason to feel hyped and I'm a well known Idoneth simp.
3) He posted about their being precedent for a same grand alliance vs grand alliance battlebox.
4) He reacted to posts about Fyreslayers getting a foot hero.
5) He told me to be less pessimistic about GW hating Idoneth.
To me an Idoneth vs Fyreslayers box does make sense. As they are both relatively older tomes, didn't receive any new units (i know slayers got some terrain and spells) in 2nd and have yet to appear in a battlebox.
It's also reasonable to think GW likes the pattern of releasing new models in battleboxes accompanied by books as this has been used for 40k 9th as well as AoS 2nd.