temporary bonuses are bad. I'd rather lower the bonus but keep it permanent. If it's only available in the Renaissance or later eras, it shouldn't be a problem. There aren't that many military wonders in the late(r) game so that could work. The thing is, such an effect would be very similar to the Brandenburg Gate which gives free experience across the board. However, there's nothing wrong with having two similar wonders a bit apart on the tech tree.
What wonder would fit such a bonus though? Mount Rushmore? The Motherland Calls? Neuschwanstein? In any case, it's the industrial and later eras that don't have that many wonders...
Having said that, I doubt we are adding another wonder right now.