Unlike the others here, I will try to give you a direct answer that will meet your definition of fairness as well as give both of you incentive to maximize your earnings in the future.
You currently have 5400€ between you and 2600€ expenses leaving you 2800€.
You currently keep 1900€ and she keeps 900€ at the end of each month splitting 68/32.
If you marry and have a child, your combined income will go down to 4900€ while your expenses will increase by 300€ to 2900€ leaving 2000€.
You could continue to split 68/32 leaving you 1360€ and her 640€. If you use this split you will lose 540€ and she will lose 260€. That's a 28% loss for you and a 28% loss for her from your end of month take home. So far it sounds reasonably fair. What about the future?
For each raise, the person getting the raise keeps 66% of their raises. If you get the majority of the raises, you keep the majority of the benefit, but both benefit from the increase. Any future increases in expenses can be split as negotiated based on who benefits from those increases. That's basically what you are doing now considering that adding a child will cost a lot of her time, not just your money.