I'm not sure what you mean by your first statement quoted above, but I would like to suggest a multiplier effect, that good ideas or good reasons get a dividend from a tax deduction to assist in the project. That is, if you can state a really convincing reason why your idea is of benefit to the world, you can add to your voluntary contribution by an amount deducted from your tax burden.
What's wrong with it is that it doesn't really reduce the tax burden, but by having voluntary donations flowing into worthwhile projects, the tax burden could be reduced gradually as those projects pay off.
I remember reading a (obviously) Science Fiction story in which every dollar of tax burden had to be allocated by the taxpayer to his choice of benefit programs. The tax form was horrendously long, but at least you had the choice of supporting the programs that appealed to you, and rejecting (defunding) those that did not!
In other words current taxes stay static. Your budget line item only gets an increase by voluntary taxation.