I’ve just found this new casual format and I really like the idea behind this.
I have not played yet nor followed the evolution of its rules but I’ve already had ideas to “improve” it, imho.
As far as I read this game isn’t about competitive so I think you should choose more restrective rules in order to make it more “realistic”.
I think the signature spell should contain the exact amount of color of its Planeswalker and it should cost less or at least the same of it; ex: Animatu and Esper Charm ok; Animatu and The Elderspell/ Fatalpush/ Dimir Charm no; Jace, the Mind Sculptor and Archmage’s Charm no.
Another cool stuff linked to that could be the possibility to have more signature spells but with the amount of your planeswaker cmc; ex: Animatu with Fatal Push, Remand and Path to Exile.
Then you should be able to cast the spell by using the same quantity of loyalty counters rather than mana (and without the +2 mana cost everytime you cast it) as planeswalker ability .
Let me know what you think about it