Clicking the upgrade button for all towers multiple times can be a bit tedious once you have a few prestiges in. Having a "Buy max" button on a tower would mitigate that a bit. This would go well with a visual option to highlight non-maxed towers
Another Idea would be a "Upgrade ALL Towers of a Type"-Button. Like it says, it would allow one to buy a bunch of upgrades for a bunch of towers without having to manually go around and click on every tower to upgrade them.

