Ro-Kyu-Bu!

Tenshi no 3P is not only just Ro-Kyu-Bu in a Jpop-rock band, they even featured the anime in one of the episodes! Both shows you get enthusiastic lolis, and both of Kanae Itou's characters here are major-supporting characters who secretly loves the [boring] protagonist. 

A teenage boy is tasked to help some little girls become better at something. In Ro-Kyu-Bu!, it's coaching basketball. In Tenshi no 3P!, it's playing rock music. Both anime are extremely cute and have amazing art. There's also plenty of fanservice in both, though Ro-Kyu-Bu's fanservice is a bit heavier. 

Bunch of cute lolis surrounding a high school boy. And both the light novels were drawn and written by the same author and same artist. 

Both based on the light novels by Aoyama Sagu and the wonderful art of Tinkle, with cute lolis that will melt your heart. P.S. Lock Your Door! 

Male protagonist who is a prodigy at a given activity at a young age is tasked with coaching a group of little girls at said activity. 

Both are about an older male coaching younger females. Both should also be watched in private, in incognito mode, and with a VPN. 

A teenage boy is tasked to help some little girls become better at something. In Ro-Kyu-Bu!, it's coaching basketball. In Ryuuou no Oshigoto!, it's playing Shogi. Both anime are extremely cute and have amazing art. There's also plenty of fanservice in both, though Ro-Kyu-Bu's fanservice is a bit heavier. 

Similar in a way where the main loli heroine admires the MC. Also, lucky MC-kun is surrounded by cute lolis. 

Five girls in a sports club, led by a man. Simple. Personally, none of them is of my taste, but "Softenni" shows a cast and humor slightly better, while "Ro-Kyu-Bu", lacking any innovation in all directions, invests most of the twelve episodes in the girls and their relationships with the coach. In addition, "Softenni" at least is more honest in your intent: fanservice and more fanservice, and, if possible, one or another tennis match. Now "Ro-Kyu-Bu!" masks this content with your innocent visual and lolis characters. Wrong, very wrong... 

Both involve lolis, sports and fanservice. Not more to say 

Both are about a group of girls who are in sport club and are trying to be the best while having a crush on their coach. Both are ecchi. 

Both are comedies with school setting and with little girls who have crush on their teacher. Both are ecchi. 

These romantic comedy series feature a well developed romance with a significant age gap. Trash which can compel one to overlook the taboo nature of the character's relationships and focus on a larger picture has never been so terrible for the soul but so good for selling you on it. 

Both are anime regarding basketball and both are awesome in my opinion. Although, Slam Dunk has less to none echii material (from what I remember) than Ro-Kyu-Bo! which might be less appealing to those seeking echii situations. Slam Dunk is a quite old anime and its drawing style is rather serious but the anime itself is not. Trust me, you'll laugh 'till you feint in some scenes :) 

Well... First things first two of the K-On voice actresses (Yoko Hikasa and Minako Kotobuki) play an important role in the anime which I found quite amusing at first. And the fact that both anime series' revolve around a group of girls trying to put what they love (Basketball and Music) before anything else is amazing. Plus the girls' personalities are awesome in both series :) I realized after watching Ro-Kyu-Bu and K-On that girls who have a lot of passion and dedication can get whatever they hope for.  

Looking for a sports-themed anime but at the same time you want to watch ecchi anime? No worries. Ro-Kyu-Bu! and Basquash! will give you what you want at the same time. 

Seinen-like shows where the boy take care of younger girls/lolis. The girls love him, but he does not fall into temptation to molest them and he protects them instead.