Stop with the season long villain showing up so damn much to face off against the hero. I just solved your biggest problem. Arrow had other problems, Flash had a few, and Legends is new so I’ll cut it some slack outside the fact that vandal savage got tired quick.
Arrow has peaked with season 2. Ra’s Al Ghul just did not click. He did not, and the more he showed up the more in compounded the problem. After the Fall episode, which I actually do love, the series was so incredibly rocky. Ra’s, his plans, and then the whole Oliver ascending just felt so… It just started to become grating. There was good stuff and good episodes but the overall arc was lackluster compared to season 2. Season 4, was worst. All the effort into making Ollie a symbol for Star City failed. Partly because most the season involved him with little victories, a lot of him getting his ass kicked, the death of Laurel and too much emphasis on the Oliver/Felicity. Dear god, I loved this show-why?
STOP DOING LONG TERM VILLAINS BECAUSE YOU HAVEN’T HAD A GOOD ONE SINCE SLADE! No, you have not. Dahrk was fun at first but you kept having him show up, kept facing Ollie down, and over and over it made him boring and stupid. Also, Merlyn-dear god stop that. Can we get a season without him?
Flash season 2… Too much Zoom. I liked Zoom, do doubt, but so much focus on beating Zoom started wearing the season down. There was so much good stuff but the darkening of Barry in some situations and the constant presence of Zoom hurt the season a little for me. The finale was strange… Awkward, head shaking. I’m not going to spend as much time on Flash season 2 because I still thought it was good, but needed more episodes free from Zoom.
Legends… Vandal Savage sucked. Just the worst. not scary, not fascinating, not good.
What you need to do is minimize your season long villains appearances. Scale it back. Have less face offs with the hero or heroes would be nice. Stop having your villains know where the hero lives would be GREAT! Most importantly, more done in one stories! More victories!