I've been saying this for a while, but I think they could take a look at nerfing her ult charge and buffing her in another area. Rez does charge maybe a bit too quickly, but if they just nerf that, she will go back to being a trash-tier pick like she was last season.
A rez charge nerf and a beam buff would be perfectly fine with me, as it would benefit the "play with the team as often as possible" playstyle that I already have with her.
I don't know what they could do to the beam. It's already very poweful. I think the change would have to be mechanical, like giving her more brevity to play without LoS. But I can't think of anything that wouldn't be extremely overpowered.
Mercy has been retooled a bunch of times. She has never been the most picked healer and was only very briefly in the pro meta before S4 and yet she garners a lot of calls for nerfs. It's like people complaining about Junkrat.
Hmm. I agree that she was too weak, but I think Blizzard took the wrong approach to buffing her. Now she is super annoying to play against in an unorganized context.
Most solo queue elo climbers are good because they exploit uncoordinated teams. It is the nature of the game. Balancing the game around solo queue is a very poor idea.
Besides, most Mercy players you're going to find in comp are going to have very exploitable game plans. Mercy is often described as being very easy to play but it's simply not true; she depends on, perhaps more than any other character, a strong understanding of positioning and the enemy team's tempo to play well. When she dies, the team dies, and that's a lot of pressure.
Case in point: The guys who hide so they can res the team after they wipe. This is a terrible strategy most of the time: rather than try to win the fight, Mercy players with this mindset engage on a 6v5, practically ensuring their defeat and relying on the prospect of getting a shorthanded pick (not to mention ideal res positioning) in order to end up on the advantage after the res. Mercy is quite ignorable in these situations, especially if you're on defense and fighting against this strategy. You can simply hang back and conservatively use ults while the enemy team flounders due to a lack of healing. If she gets a big res and you've still got an ult advantage, you have a good chane of wiping them again.
Many, if not most, Mercy players in comp believe that hiding for a big res is a good strategy. I say this both as someone with a lot of Mercy playtime and a player at 35xx SR.