Reverting/changing rift herald would have a massively positive impact on the state of the game
Right now we have games ending too early, personally, I feel that rift herald is a HUGE part of this problem. There are plenty of other issues, don't get me wrong, but I think the changes to herald perfectly encapsulate the late season 7/preseason issues. Primarily, paper turrets and top lane feeling as though it lacks purpose.
One of the biggest issues with herald is that it chunks whatever turret it hits first by 50%, which is massive. It doesn't matter if it's a front or inner turret, it's losing half. Herald stops to fight minions which is a great idea, it helps to make it so you can't just throw her anywhere in lane and get a free tower. You have to push up the lane to create an opportunity for optimal use. What confuses me is that if you just wait for a wave to pass and drop herald, she goes straight for the tower, still chunks it and allows you to take it pretty quickly. Completely bypassing the necessity for pushing the lane and giving the enemy team much less time to respond to a guaranteed lost turret. It's a small change but I think herald should move back to the minion wave if it's placed BETWEEN enemy minions and the tower, instead of focusing on the turret.
I also firmly believe that the initial damage on herald should scale based on the kind of turrets it's knocking down. I don't think it should be able to instantly rip 50% off of an inhib or inner turret. It just doesn't make good sense.
Honestly, I think herald would be better if it were nuked from the game entirely, but that's just me being salty. I think another solid possibility is to make herald a buff that gives some helpful stats. I think this could bring some focus back to top lane. Herald itself on map is a ducking nightmare. However if we can give a top laner increased pushing power for a few minutes or stats to help win/turn lane, it may put some focus back on top lane again. Especially if those stats are something that help increase their damage OR staying power in teamfights. So like a mini personal baron buff.
Either way, I think that the most positive change you can make in preseason is changing rift herald. A team that has the advantage doesn't need a free tower, and a team that's getting dumpstered isn't coming back off the free tower unless it's an inhib, which is somewhat unlikely if, again, they're getting dumpstered. Right now Herald is just a win harder mechanic, the same goes for baron. Teams that are ahead are given too many advantages to make it even easier to close out games. This is the central issue with season 7/8. We need to change that, map monsters would be a solid place to start.