Just a simple change:
Add the option to upgrade buildables to Tier 3 in the campaign.
First, Many missions have limited/specific placement of turrets and traps, that provide a tactical advantage but often become fairly useless at higher levels of difficulty (including larger player groups) due to increased spawns and stronger enemies. Being able to upgrade turrets/traps means we can not only make them stronger, but we can make them more survivable (since higher tier turrets get shields). Shields are really useful for turrets, because they allow them to take small amounts of damage without letting it accumulate.
Secondly, we have plenty of Legendary items that only function on Tier 3 buildables. Currently, this means if you don’t play PvP (or can’t ever reliably get into matches, courtesy of your local playerbase), those legendaries are useless. Nothing should ever force players to play one way or another, the game is meant to be here to facilitate our enjoyment.
We already have Tier 3 for the three types of turrets, so even if they don’t want to add functions for T3 traps (or even drones), it would be much better than nothing.