Its been forever and actually imo she should return to main cast., Many people feel there was left much more to be done with character to grow as person and trainer never becoming water master. And by tying her into E4 arc would make her both relevant to story and influence Ash growth by giving him insight in how E4 works getting to know their strength and weakness revealing to him more about ways how to earn right to challenge and beat them.
Granted some will say that would be bad fearing writers would "ruin"her,. But thats very subjective. For example some people feel Ash was ruined in Hoenn and Sinnoh, but others loved his character there enjoying in his personality and achirvements. Some hated Brock in Johto, Hoenn, Sinnoh(although he was shafted in Sinnoh too much) while others loved him.
Some feel Team Rocket became worse in each new saga, while some think exact opposite finding how they became only better loving when Jessie became coordinator. In BW Japan audience disliked them including voice actors while western audience liked them becoming more serious and agile.
Everyhing is very subjective so in reality until you watch it for yourself you cannot really form good opinion about this.
But in nutshrell im all for Misty return. She knew how to fulfill Ash and bring more fire, making interactions between chasracters filled with more energy thanks to her spunky, romantic, selfreliant and optimistic, caring full of playful sarcasm, hotheaded attitude and unselfish loyalty side. Knew how to battle, had many unexpolored things about her motives and past(parents, fear from bugs, why she wants to become water master, in what way to achieve it, her own fears and doubts)opens whole chest of unexplored possibolities to do many new, cool things with character.
Deserving love and recognition.