The Seraphon army is great rules and model wise (mostly), but the "memory being" fluff is IMO quite shitty.
Thematically speaking, we already have forests/jungle dwellers (Order Draconis, Serpentis, Wanderer, Sylvaneth, etc.), and also Sky dwellers (Kharadron, Stromcast, Star Drakes in Azyr).
So their "niche" fluff needs to be full Space dwellers, with still better magitek than KO.
I like to imagine (based on the Blog des Kouzes conversions) a pure "Aztec alien reptilian" faction, from space, still flesh and blood, living in space ships above Azyr.
This way, they are not the "celestial angels" the Stormcast and Tyrion aelves already embodies, and will be linked again to the Lizardmen Exodus and space faring Slaan fluff they always should have be related to.
They would still be defending Azyr, they would still bypass the need of Realmgates, and the summoning and teleportation mechanism they have in the rules would just be them being beamed down form their space ships.
The fact all original Lizardmen from the World-that was died during the travel to Azyr isn't a problem : the Slaan can just recreate new generations from swapping pool in the space ships. Maybe their remain just one pool, so they cannot have endless legions.
And justification for new tech for basic troops would be simply that the non-Old-ones, pure Slaan new Grand design made the surviving Slaan realize they need every weapons they still have access to to counter Chaos.
Also we need to link the fluff of the Infinity Gear (and the time cannon) with the Seraphon.
TLDR : LAZER DINO FROM SPACE MADE REALITY