I too have been struggling to come up with a mechanic to make combat more realistic/relevant. S-P-S and playing cards both seem to fall short in what otherwise is a very immersive game experience. And while I am not completely satisfied, I have come up with some ideas I would like to share.
The first idea is really just a glorified version of the playing card mechanic. It would require anyone who wants to openly contest another player to challenge them to a duel. Depending on the setting this would take on different forms (eg, a gunfight in the Wild West, a sword fight on a pirate ship, etc...) however, all would play out in a similar fashion. The players would approach the host, who would present them with a choice of weapons (actually two cards with identical faces depicting a gun, or sword, etc...). The players choose their weapon, the challenged player going first, and then they turn their cards over. One card is the winning card, the other - the losing card. Players can then act out the results if they wish. As I said, not that much different from the playing card mechanic, but I think the simulated action of choosing your weapon maintains the immersiveness by some small degree. I would like to add a strategic element to this if I could, and give some characters advantages, but have not yet figured out how to do so without affecting the balance of the game.
My other idea applies to assassination attempts where the would-be murderer tries to do away with another player in a stealthy manner. The tic tac idea that someone else had in this thread is wonderful, and reminds me of the inspiration for my own. In one game I played, weapon cards were used. These cards were meant to stand in for various weapons and were to be used as a sort of currency to commit murder. In most cases, all you had to do was corner your intended victim, and when no one else was looking, hand them your weapon card. And WHAMMO! They were dead. A little too easy. We had a stack of dead bodies a mile high! The only weapon card that was different was the poison card, in that it required the player to wave the card over food or drink that their target had to then ingest. This requirement both made the poison more difficult to use and emulated the real act of poisoning someone, and I liked that. I have since come up with other challenges to using other types of weapon cards- with fantastic results! Weapons for stabbings can only be used if the target's hands are otherwise occupied (so they can't put up a fight). Weapons for strangulations can only be used if the target is approached and surprised from behind. Point blank shootings are easy to carry out but result in the target yelling 'bang,' quickly directing players within earshot to the scene. Long range weapon cards have a hole in them through which players must track a target who is at least ten yards away for ten seconds without anyone or anything obscuring their line of sight. All of these rules may sound complicated, but they are clearly and succinctly written on the cards themselves, so that understanding how they work requires no memorization and is as simple as can be- even if successfully using them is not!
As weapon cards can only be used if there are no witnesses, players are very much on their honor. In the games I have run, however, I have never had a real problem with players trying to cheat. I think that is because there is so much freedom to do what you want in these types of games that people don't mind the few rules there are. Plus, I think the challenges that are posed, like those associated with my weapon card mechanic, are fun and only add to the immersive experience!
I hope these ideas are helpful to some and look forward to hearing anyone else's!