That's not what I meant...
High-end GPUs all have additional power connectors via the Power Supply. Here's how it works out for power.
The PCIe x16 slot: 75-watts
First 6-pin PCIe power connector: 75-watts
2nd 6-pin PCIe power connector: 75-watts
additional 2-pin connector (6+2 pin PCIe power connector): 75-watts
Total power: 300 watts Maximum power usage.
Each card is only capable of having a 6-pin PCIe power connector and another 6+2 pin PCIe power connector, so all together, each card can only use 300-watts total power.
Now, in terms of bandwidth between the GPU and the motherboard, we haven't hit the limit yet, and there's no reason for them to change the slot type. PCI express is an incredibly efficient slot type, and I don't see that being phased out anytime soon. If they need to change anything, they'll more than likely change either the bus speed or the bandwidth before they actually change the slot type.
No need for concern at all.