Getting the base Malign Sorcery box is pretty much the best route. As for something faction-specific, the Daughter's of Khaine have a snake endless spell that would look good painted in jungle colors with water or slime coming off of it instead of blood.
Which factions have their own Spell Models? (I thought Seraphon did! Didn’t they release some spells when they released the pyramid terrain thing?) I think a Seraphon version of that barrier of crystals would be pretty easy to scratch build if it was made to be a barrier of logs and trees that had sprouted from the ground.
Seraphon don’t get their own endless spells because their gimmick is their mastery over magic meaning they can bind (and bring as part of a list) any faction’s endless spells, which I guess makes sense even if it sucks to not get unique ones. The new pyramid is a terrain feature, like the sylvaneth wyldwoods or the new Lumineth floating shrine
not true we can bring generic ones but the faction ones are Beyond us. We do have to spell that can control other people's spell for a turn now