There are people like heisendong and shaclone who have been ranked master/challenger by only playing specific champs. You can make anything work is what i would tell you. If you like leona keep going.... BUT
At higher ELO's she isn't as good. you literally can't engage on thresh, janna, alistar, tahm kench, morgana, sivir, draven, or vayne. You will never touch the adc in any of these lanes.
ADC's are very mobile right now. Good ADC's will dodge the ult stun almost every time.
I mained jungle and support up to D5. I played leona through gold and it was the best thing ever. People make less mistakes in the higher ranks and i found picks like thresh and braum to be better all around.