To clarify, most proposals I've seen have involved one of the roughly equivalent options of a separate flat tax specifically to fund UBI or an increase of the same amount across all tax brackets.
I have seen some proposals involving replacement of taxation generally with a flat tax coupled with a UBI - sold specifically as being able to still produce an overall progressive curve with the simplicity of a flat tax. While I think those are both positive things, I find this oversold - most of the complexity of taxation has little to do with brackets, and while the resulting curve is progressive it is only a particular family of curves and it's plenty possible that the optimal curves (by whatever metrics we assign) are not in that family.
I have seen some proposals that involve a progressive increase in taxation; these have been (in my recollection) fewer and less detailed than the first category - mostly, I expect, because they are a bit harder to analyze.
