What is needed is not another Griffons, but additional options that have less Faster Cast, but give more choices to the game.
For example, faster cast on a rare Staff is outshone by a rare Orb and Shield combo, adding a higher level suffix of faster cast that only applies to staves would open up more options for Casters if they are willing to forgo a shield. As high as 60% Faster Cast on a Rare staff would still be outshone by dual Spirit, but it would slightly edge out a Rare Orb and Spirit shield, thus depending on what skills roll as to which option is better.
But probably the best way to approach this would be to add Faster Cast to most Caster crafting recipes, carefully keeping the possible rolls below current best in slots. For example, in the link I suggest 5% to 10% Faster Cast on Caster Gloves, which is half the Faster Cast you get on Magefists.
The aim here is not to replace or surpass current meta end game gear, but to provide more “good enough” options for people still gear hunting, and to give niche builds more options as well.