I would like to see time travel be used a lot less after future. I think once he's learnt from his mistakes in Flashpoint he'll finally realise that he can't keep messing with it. Time travel has been a major point of the last two seasons.
There's a lot more about the Flash that needs to be explored. I'd still love to see more about the speed force and it's different uses. I'd love to see a season dedicated to the Rogues - maybe after his experience in the Legion of Doom this year Captain Cold returns to Central City clear in his mind he needs to build a team and we finally get the Rogues.
And finally I think the Flash is the show where we really need to see the Justice League born. By that point Arrow I'll hopefully be finished and we can have Green Arrow and Flash forming the team and fighting a threat in Central City that requires the league. It has to be on the Flash because Supergirl is another universe and Flash has the most meta's. Other than Atom I really do not see any other heroes introduced on Arrow even being considered for the league. Vixen is from her own show and I would have took a fully powered Black Canary but we'll never see that now.
From Flash has given us so many options already it would be crazy to think how many potential league member's we might have by the end of season 4. Flash, Vibe, Hawkman, Hawkgirl, Firestorm, Atom, Vixen and Green Arrow. Imagine the seven of them fighting a huge, season long threat.