First of all thank you for the suggestion, these are always welcome and cool to read.
Before reading your last paragraph, it looked pointless since you're talking about situations where your team is base-locked, and indeed if the enemy team controls the battle field entirely, then your engineer will probably get demolished trying to harvest anything.
The idea of crystals outside the battlefield is a cool idea, but I see the following risk: the enemy team is already ahead of yours economically since their harvester is at work and yours isn't. Now imagine if the enemy team also dominates those tunnels and caves where the crystals are -> then their economic advantage would just get bigger.
Also I do not really see what is wrong with earning credits from repairs. When you're base-locked, there's usually a continuous pounding from enemy tanks onto your tanks and buildings, which makes it quite reliable. Think about maps like Field and Under. I rarely do it for the credits only, I like repairing tanks, you usually cannot win games without repairs anyway.
In a way I am also in favor of not bringing too many gameplay changes compared to original OG, because if you make one such change, then the door is open to other significant changes. And in no time you can end up with a pretty different game.