Well a way to make a high quality concept is like @Musketeer said to put your mind to it, really think about the characters skills how the character will work, how do you want him/her to work? And try to be unique with the concept like probably add a new status or something or make a unique mix of effects or a new effect entirely but at the same time don’t make it to overly long that’s a mistake I’ve done multiple times.
Friendships, friendships arent just random characters together, you gotta really think about it in like a irl way, would these two possibly be friends if they were in the same universe?
The effects of a friendship are of course the hardest part of friendships, I base effects of friendships I do on how possibly the characters would work together during the friendship campaign or something like that.
Basically take your time and don’t spam concepts that took like 5 seconds to think of and 30 seconds to make, try to really think about skills, friendships and the characters your working on.
And if you really wanna see some great concepts that introduce some of the things I talked about above I suggest concepts made by @Shining_Diamond , @Card_Brigade , @Filadae_Djaq , @Gjmguy , @LetsGetDangerousCousins , @Musketeer , @Champion_David , @Kinokan , @TherMasterStitch
Each of those people are amazing concept makers who have some amazing concepts.
And if you want you can check out some of my concepts but they’re really mediocre,
Hope this helps you with your concepts! 
