Few months ago I tried to convince one of my cousins to play FGO. So, I tried to explain the game mechanics to him, showed him the skills and how they work.
After seeing the skills of few servants, his answer was, "they are all similar" and became uninterested in the game.
And he was right. All the servants have similar effects, with shuffled skill set. All the top tier supports and dps have similar skills with different card types. In fact when a servant has a unique skill or np, it's a big deal and a topic of hot discussion.
I think the reason of this problem is the mechanics of the game. There isn't much option for the developers to do anything different within this mechanics. All they can do are give higher np battery, higher attack/mana bust, rainbow effects and other old stuffs.
Everyone agrees that the game is outdated. But the game is so outdated that it can't even be modernize without a major overhaul. But at that point I think they will just make a new game instead of doing this.