So far, the first two seasons of the Flash have been amazing! The show has so much potential to do well going forward. But it also risks making the same mistakes that Arrow made after Season 2. In my opinion, Arrow did an amazing job up until halfway through Season 3 when Oliver was defeated by Ras al Ghul. From there, it just went downhill.
One of the major reasons why this happened is because there were too many characters involved. With so many characters and so little time to do the writing, the plot and the dialogue diminished greatly. An example of this is Felicity. Prior to Season 3, she was this extremely supportive character in the series. She was not easily angered and just seemed to make everyone happy. Now she is one of the most frustrating and annoying characters to watch. She constantly flips out at Oliver and broke up with him in a way that practically turned the show into a soap opera. Instead of seeing the team support each other, a lot of times, we see them cause great emotional ache towards each other. Also, the fight scenes in Arrow have just gone way downhill since more characters got involved.
As Iris and Barry get back together, they should make them a happy couple, not a couple that is constantly having problems. So far, this is what I have seen and liked in Season 2. I think she should stay this way, being there for Barry when he needs her and not constantly giving him crap the way Laurel and Felicity gave Oliver. I would also like to see her travel more for her job as a journalist or explore problems outside of the city, which could lead Flash to those places as well.
Another point to make is that as the Flash starts to add more characters, they need to do so prudently. I do not think that Jessie Quick, Wally West, Jay Garrick (who I think is the man in the mask) and Vibe should constantly help the Flash fight metahumans. In Arrow, with Thea, Diggle, and Black Canary constantly showing up to the battlefield, there is too much going on. It also takes away the novelty of seeing them together. Seeing Barry, Jessie, Wally, and Vibe fighting metahumans together should be a treat for the audience, not something they see every episode. It should almost be like seeing Barry's father or a character from Arrow - not super common, but extremely fun when you see it. Perhaps a better model for this is how Firestorm was presented throughout Season 1. He wasn't always there but he was there when he needed to be.
In order to see more of these other heroes individually, they could take Barry and have him go to another world or into the future and then have these other heroes take over for a little bit. They need to do something to avoid having a team of heroes constantly working together because that is difficult to do on a TV show.
Next, I think they need to take a break from having an evil speedster as the big bad. I think next season, they should go with someone like AbraKadabra, a man from the 64th century who travels back in time and uses advanced science that appears to be magic.
I think it would be cool to see AbraKadabra as the big bad of Season 3 and to see him start to form the rogues together. I think that he should have Shade and MirrorMaster work as his henchmen.
I think AbraKadabra could give the writers a chance to fix the flaw with the Season 1 finale. If Eobard Thawne's Doctor Wells was really erased from existence, then Barry would have lost his speed and any memory of the event, Firestorm would have died in episode 13, all the metahumans wouldn't have existed, etc. If the time paradox was supposedly fixed through Barry and Firestorm, then why didn't Dr. Well's Eobard Thawne survive. I think that it would be cool if AbraKadabra used technology to give the appearance that they erased Thawne from existence and made him actually survive the event, and then opened a wormhole to see . Then they could bring in Dr. Wells as the Reverse Flash (Barry's former mentor) and have Earth-2 Dr. Wells get a chance to meet him. That would be fun. If they do something like that, we could see different versions of the Reverse Flash in Season 3.
The final thing I think they need to do is implement time travel into the show. As we saw them do in episode 17 of Season 2, Barry Allen visited his younger self. I think we need to see more evidence of Barry Allen from future seasons doing that more often. Getting more hints of what will come down the line for Barry will be very cool. This could allow them to hint at potential future big bads (I would really enjoy a lot of references to the Black Flash). Additionally, it would be nice to see him go on a few trips to the future and maybe see how he created Gideon, etc. That would be fun.