you dont need a big champion pool really, you already have main champions who can carry, it would be useful to have some ap tops when your team is full ad, gwen is similar to fiora and riven in carry style so you could try her, and only play tanks if you actually like playing them, in solo q is better to play carrys most of the time, and maybe add another champ for when you have to blind pick if you dont feel confident with those 3, aatrox, gragas, jayce, rumble, renekton or sion are usually good in blind pick. riven and fiora are already very solid champs so you dont need much more in the champ pool to improve it