The problem with wanting Zaladane to be Lorna's version of Sabertooth to her Wolverine is multiple which I don't think those in favor of the prospect have really gamed out. It could very well have happened as Claremont wanted to revel both Lorna and Zaladane as Magneto's daughters on Genosha and then have Zaladane destroy the island.
Cassandra Nova in the end did the deed and Nova stayed dead. Austen also planned on reveling that Nova was playing a direct role screwing with Lorna by using Carter something that the last page of Nurse Annie leaving showed.
Cassandra Nova would frankly work as the mutant antagonist she needs. She is high enough profile. They have that connection to Genosha and its destruction and even screwing up her wedding. Nova will be in Deadpool 3 and therefore become a household name unlike Zaladane and they have a built-in connection.
Nova will be better known to the masses very soon then Lorna. That is a very good thing if Marvel saw fit to make her a rival for Lorna. Sabertooth on his own separate of Wolverine is quite popular for instance.
Right now, the House of M family dynamic is screwed up to high hell because of the decision to not have them be blood related probably due to waiting on the MCU. That means they have to play up their good feelings for each other beyond the max to justify them acting like a family. Once the retcon comes down I can see Lorna having real disputes and rivalries with Wanda and Pietro. House of M being a family again will be more popular then the Fantastic Four when it happens as long as it gets the writers it deserves.
In terms of the x-books Cassandra Nova was the replacement for Zaladane very thematically and while she doesn't bring the allure of having a new 'rival sister' to Lorna's storytelling Cassandra Nova vs Polaris has the mountains of background set up already done for it.