Out of Misty pokemon here are my guesses who could have eolved by now:
-Staryu:not really,there wouldnt make any sense to evolve it since she has Starmie already.
-Goldeen-not sure about this one,prob.not.
-Azurill:still baby pokemon,not expcted that he battles a whole lot.
-Horse:this would be best bet.Im sure he might have evolved to Seadra,maybe even all the way to Kingdra.
-Psyduck:he will probably satay that way forever,being one who is responsible for comic relief.
Judging by past info,the only pokemon Misty mostly uses in gym are Staryu,Starmie and Corsola.Though that could likely change in meantime.
However its needed to point out how pokemon dont always evolve by training and battling,sometimes evolving through other ways.Rarely,but it happens as we get to see with Togepi.
When it comes to Mistys progress as a trainer:reason why she didnt battled more back than when travelling(though there were probably off screen battles too) wasnt because "Ash was holding her back",but more because writers didnt focused much on her aswell not having same amount of experience in writing,like they gained later on learning on past mistakes.However it cant be denied that she improved her skills in Jojhto days compare to early days when she was just starting.
As time moves on,writing becomes better,which Dawn case clearly showed being first time in history that character wins big competition(GF) on first try rapidly improving as trainer(this case coordinator).
In Mistys case,now at gym true she battles more now,but its quality not quantity which matters and considering how majority of challengers are medium level and rookies,potential for further improvement is being held back.Its a well known thing how you cant reach high class trainers level which Misty aims for(like E4,champions etc) as a gym leader which explains why none of gym leaders is that strong,and those who made bigger progress like Wallace did that by leaving gym to travell.Speaking of Mistys goal,not to mention there were tasks which are needed to be completed to become water master,which was evidently showed during Whirl Cup.
Apologies for this mini wall of text,but i wanted to say a word or two about said subject which needed to be said.
...hm, if I think about it that way, Gen 4 Water Pokemon, perhaps she has a Mantyke? or.... what if they let HER debut a Gen 5 Water type?
I thought about it too,Mantyke would fit her nicely,same goes for Finneon but due to lack of info who knows what changes in her pokemon roaster happened in meantime.
About Misty debuting 5th generation water type,that makes sense and it could be posibillity next time she returns.
Misty's sisters made it apparent that they wanted little to nothing to do with Pokemon Training so rasing the Pokemon in the Gym to respectable strength has pretty much been Misty's responsibility.
Lily and Violet,not but Daisy showed interest in pokemon and gym which was showned in "Luvdisc is a Many Splendored Thing",spending most time with Misty helping her about it,which opens posibillity of her providing decent substitute if Misty leaves gym.That will of course only happen if writers decide to do so.
When it comes to other pokemon like Seakings or Dewgong which are at gym,its debatable under which possesion they are since Misty sisters are tehnically gym leaders who are running out of obligations.