Riot send me an email must of been meant for you, It reads there are 120-150 champions Your Fix has already be predesigned into the game.
It's Called
** Learn and Perfect more champions. I
Seriously not trying to be a Dick. Just being honest If 25 isn't a big enough Champion Pool for you, You'll simply have to expand it. Riot isn't gonna buff/nerf champions bc you can't counter pick or whatever. That's all on you and you know it. I'm bronze 5 but even I know the answer to that question. Its to practice and playmore. Not trying to tell you higher ello guys what's best, but this one was to simple.
Ps: Yes I know keeping in practice with those champs (25) takes a lot of ur time, but you are only D3 so until you reach challenger you can't say its the system holding you back from rising. Gotta hit the roof first :p anyway good luck.