I think it's mostly already there. When formatting a text box or shapes, simply enable Background = On and define both the Fill Color and the Transparency for the selected object.

You can even use any of the CSS-3 color codes (incl. RGBA, HSLA) in combination with conditional formatting and a measure. E.g. for a hardcoded (i.e. unconditional) color create a simple measure like:
SemiTransparentBlue = "RGBA(0,0,255,0.5)"

Then select the [fx] icon next to the color selector, choose Format By = Field Value, and set Based on Field to your (e.g. SemiTransparentBlue).

For real conditional colors your measure should be a bit more advanced though, like:

ConditionalColor =
var _value = [] /* the value to compare */
return
SWITCH( TRUE () ,
_value < -0.1, "RGB(255, 0, 0)",
_value < 0, "lightpink",
_value > 0.1, "HSLA(120, 100%, 75%, 0.5)",
"" /* else (0 >= value <= 0.1) */
)

Note the blank color code as else parameter for full transparency. Very useful for table visuals with alternating white and grey rows when no conditional color should be applied for specific values.

See also: https://docs.microsoft.com/en-us/power-bi/create-reports/desktop-conditional-table-formatting#color-formatting-by-field-value

Still, allowing 8-digit hex codes as in Power app's ColorValue () function would be more user friendly. So I voted too :-)