Not sure, but I did like the idea of moving the green base around, but then the balance of just always dragging it around vs leaving it is tough. Although dropping off discarded tanks randomly or even from past runs could be good if they held small bits of energy or crystals, Loot them for items.
I thought a little more about this and thought of maybe giving the payer a shop with upgrades but the player can only carry a few upgrades. Carry more energy, get more energy when recharging, carry more crystals, get a better price for crystals.
I was also thinking of allowing players to sell their crystals for money, money would effectively act as the players score. So basically they could turn in their crystals for upgrades, energy, or score.