This seems like a perfect candidate for a mobile game (particularly local multiplayer on a tablet sounds like fun) but I'm guessing that is not on the table due to publishing issues? Is this strictly a desktop game? Depending on the control system you had in mind web may be an easy port (just something like itch.io would be fine). Throwing this out there now because I am assuming that you probably had Unity Matchmaker in mind but there is no WebGL support for that so if you wanted to also do web you may want to go with an alternate like Photon.
As far as some design ideas to throw in the mix, maybe hitting certain pinball elements in the field will throw power ups (or downs) to one of the players/paddles. For example, shrink paddles, grow paddles, safety bumper, slow motion, frozen, spawn extra ball, etc.