My idea (which may be repeated, but I searched and didn't find it) is to be able to add several builders to a same building upgrade. How? You start upgrading a building like you would normally, but once you start the upgrade, there will be an option to add more builders. As a drawback, you will not be able to cancel the upgrade and/or remove any builders.
I will explain how this could work:
1 builder = same as now
2 builders = 2x faster
3 builders = 2.5x faster
4 builders = 3x faster
No 5 builders option
This can be really useful when you have few buildings left for your townhall, or if you couldn't farm for a few days so you will not put any buildings to upgrade, or when high in cups and want your defenses to upgrade as soon as possible so you don't get wrecked xD
PS: I don't know if there is any grammatical or spelling errors, but I'm not English and don't know know that much
