We did have something similar to what you described before we transitioned from deductibility to tax credits. It had essentially the same effect. If you were in a higher tax bracket, you got a greater tax incentive. This was seen as regressive and as being unfair to donors.
One solution for that lies in what you propose. You would treat everybody equally, regardless of income, so that if you gave a greater amount, you would get a greater tax recognition. There could well be something to be said for that approach, although the exemption of capital gains tax on donations of land and private securities would achieve a similar outcome. That's before the committee from a variety of submissions.