An engineer class that disables the primary weapon just like the infiltrators, and instead gets to use a new different suit that boosts the ability to repair by 10% or 15% faster or more hp points.
As good as the idea is, disabling the primary for a "slight" boost in repair speed is a huge price to pay. I get what you want to do, remove something to gain a boost somewhere else, but in case of the Infil, he gains a near-permanent stealth. For an Engineer, a fully-upgraded repair kit repairs around half the health of a generator, a +15% boost wouldn't make a great change, and if 2 Engineers repair it the boost is basically useless. It could get some flavor in the case of a driver however.
Considering the price, I think the boost could be higher. For example removing the heatbar from the repair tool.
I just thought of this while waking up, and didn't went deep into details/numbers and such, but : Why not making it so it acts like the Engie passive cert line for aircraft ? In other words, like a nano-repair ?
