1. Character development for Oliver Queen. We almost always see Oliver as the "Arrow" (whether or not he's wearing green leather). Whatever happened to re-claiming Queen Consolidated and using the company to do good? Whatever happened to Oliver being broke and possibly facing the fact that he couldn't even afford to be the Arrow any more? What happened to the idea that Oliver could do good both in and out of the mask? I don't want a bunch of boring boardroom scenes, but one of the most interesting thing about superheroes is the duality in their character and we have none of that with Oliver. He doesn't have a job. He doesn't have an apartment. He barely even has a secret identity any more since almost every other named character on the show knows who he is (or is wilfully ignorant about his identity).
2. Smaller cast. I know, it's cruel, but they need to think of some way to reduce the cast. They don't have to kill off anyone, but they need to consider writing out characters for 3 or more episodes at a time to allow all of the characters a chance to develop. Basically, they need to take it in turns. Diggle can head off on his honeymoon or an ARGUS/Suicide Squad mission for a while. Oliver can send Roy to Bludhaven on a mission to gather intelligence about some threat. Felicity can be called to Central City to help with some problem or go and visit her mum in Las Vegas for an extended break. Laurel can leave to go and train with Nyssa or Lady Shiva or Katana or someone else for a while. At the moment, none of the characters are being fairly treated by the writers because they're spread far too thin trying to develop all of them which is a near impossible task for so many characters.
Oliver's "revelation" that his primary motivation for facing Ra's again is because he can't stand the humiliation of being beaten shouldn't have come out of left field. We should have had a better idea of just how arrogant Oliver had become and how selfish he had become. There were little hints dropped here and there, but I don't think it was sufficient to explain such ... pigheadedness.
3. Flashbacks. If these are going to continue until the end of season 5, then they need to be better. The story needs to be compelling and preferably it should tie into the present day action, whether directly through a continuing storyline or thematically. While we may be leading up to a somewhat interesting "revelation" for ARGUS, Maseo and Tatsu this season, I don't think countless boring flashbacks is sufficient justification for whatever revelation they do have planned.
If they can't come up with any more compelling flashbacks, then drop them. Or instead of having an entire season-long series of flashbacks, just use flashbacks if and when the current storyline justifies them. I feel as though everything that we've seen with ARGUS, Maseo and Tatsu to date could have been captured in flashbacks in one or two episodes, instead of being spread across 10+ episodes. Not only have the flashbacks been dull, but their continued forced inclusion cuts into the available screen time for the cast of characters in the present day storyline. If they'd devoted some of the time allocated to the flashbacks to trying to expand the character development of Oliver, Malcolm, Roy, etc this season, then perhaps it wouldn't feel so disjointed.
Honourable mention: Have fun! Sure, "realism", "gritty", "dark", "mature", etc are all great buzz words, but at the end of the day this is a TV series based on a comic book. Stop running from that fact and embrace it. People can have "powers" on this show. Laurel could have been in Central City and she could have developed the Canary Cry. The Lazarus Pit could have been used by Ra's to revive Oliver after their battle. The news could have started calling "Arrow" the "Green Arrow" especially since there is now a "Red Arrow" (and sometimes a "Black Arrow" (i.e. Merlyn)) running around the city.
What do you want to see *most* on Arrow? And a pre-emptive chastisement on those of you who go beyond three things. 
Since that's too far out there, better relationship stuff.
