I think that if they are going to bring back Mainz, they would bring back Trier too. Prussia is not going to like this, so you may end up with a Prussian Netherlands, or even Prussian absorption of all of Saxony.
A Prussian Netherlands (ex-Austrian, future OTL Belgium) is going to upset the Dutch, but the main ones pushing for rewards for the Dutch are the British and the only people who can give territory seized in the last couple of decades back to the Dutch are the British, and they've done that as far as they are willing. They could compensate the Dutch with formal dominion over the presumed Portuguese East Indies (which would not happen in OTL until later in the century), and maybe Portugal gets a rose-coloured strip on a map decades in advance, showing Portugal as sovereign from Angola to Mozambique, though with no reality in the the middle at this juncture.