Misty doesn't "need" to do anything. Just like she didn't appear in DP at all, its very likely she won't appear in Best Wishes at all. She got her appearances back in AG.
Except Misty definitely deserves to return again.
Misty was character who was never fleshed out enough compared to ones like May or Dawn with several things in her story being left unresolved and at standstill.
From her goal of becoming water pokemon master,potential to find more about her past and family,several pokemon which were never used to full potential(like Psyduck,Horsea,or Goldeen)etc.
Unlike other girls Misty didnt received enough development as character nor her story ever received closure having enough stamina and potential to work even as main character once again.
She was without doubt very important character during original series who helped Ash to grow in better trainer contributing to this show popularity achieving huge popularity and iconic status and to get rid of her for good pretending like she never existed would not only be wrong but also disrespectful from writers side both toward character and fans.
Impact she left on anime and fandom was also big with many people missing her worldwide wanting her back and because of being part of original trio who made this show becoming most recognized group to this day she will always have chance to return.
Same applies to Brock.
Lets not forget also fact how Misty is character with longest gap between her appearances who was absent longer than any other character from this show(including Tracey).
Because of everything above if anyone should have priority in coming back before others it would be definitely Misty.
The show has now gone on for so long after the end of Johto that Misty has retroactively become a short-lived character in the series. She didn't even make it to the mid-point of the series. I think she's only in 40% of the shows total episodes now.
Yet she is longest lived female protagonist with her popularity as character and impact she left on show whit-standing tooth of time.
Since Kanto won't be remade in the games anymore, they passed up that chance when HGSS came out, which was the last semi-reasonable excuse for Misty to appear.
Only if writers intended to use her as one of candidates to help advertize new games.As it can be seen with her cameos in Hoenn she didnt advertized anything but still find her way to return and story-wise there are ton of plausible reasons to have Misty back.
Like:-to advance her goal
-help new characters to get over their problems contributing to anime plot(similar how May did by helping Dawn to get over her confidence issues).
-catch new water types which arent possible to obtain in Kanto(like its case with Unova pokemon) and gain new knowledge.