Well, to start, what's not fun for you? For me, the AI hold this game back more than the actual sliders/gameplay. I actually think 21 is very enjoyable when my sliders are working (different discussion) outside of the AI blunders. That being said, I've identified what I like and don't like and have used sliders to try and mold my experience to something I like.
So, what part of the game play are you not liking? Game speed to fast/slow? Passing too easy/hard/stiff? Shooting too easy/hard? Goalies too quick/slow? Goalies reacting too quick/slow? Too much open/closed ice? Too many guys stumbling? Too many/little big hits? If you could provide a certain direction/list of things you're looking for, I'd try to help you out.
In my honest opinion, NHL 21 is much better than 20 when it comes to goalies and neutral zone pressure. If you leave the sliders are default, might not be a ton of a difference goalie wise, but I've actually been super impressed by the way mine are playing and the AI gives a pretty good fight in the neutral zone now with the right sliders. If you're willing to tinker and try things out and NHL is you most played game, the sale for $30 should be something to consider.
Let me know what you're looking for and I can try to help. My experience will be heavily based on 21 just so you know, but a lot of these slider changes should carry over between the two games.