To be fair writers had couple of good opportunities to bring Misty back if they wanted like:
-we all know her ultimate goal was to become water master,so coming in DP for some water tournament(similar to Whirl Cup)to advance her dream would make sense
-or to catch new water tye which cant be find at place were she lives being possibly involved in some plot like Gary helped gang about villains.
-appearing at Pallet town was also plausible idea but writers missed on that
-considering she appeared again in games(HGSS remakes),writers could have use her as one of candidates to advertize them.
-some say she could come for Sinnoh league aswell to give moral support to Ash and perhaps even have small battle with him
Now that i think about it in that interview with mr.Hidaka awhile ago he also mentioned how if Misty rerurns she might come for some tournament like May did or have something to do with contests but not doing them,but it seems writers passed on that aswell.
As we can see from examples of Fantina,Jasmine,Candice etc, gym leaders can leave gym from certain period of time to travell for various purposes so its not like it would be problem if Misty left gym aswell.