I think rather than a combat badge, why not a modification to the trades badge?
To qualify, you would have had to serve in a combat arms subunit (meaning that drivers, medics, clerks, etc. would qualify, but only if they were directly employed in, or attached to, an infantry company, pioneer platoon, armoured squadron, engineer squadron, armoured or recce squadron, or artillery/air defence battery that was employed "in the field").
The trades badges already identify what trade a member is employed in currently. Why not modify the badge as follows:
If employed on a peacekeeping mission for one month (or if wounded/injured during before that qualifying time elapses), your trades badge gets a UN blue surround (ie piping around the edge of the badge).
If employed on a peacemaking operation (Somalia, IFOR - ie not peacekeeping but an operation short of full blown combat), you get a maroon surround.
If employed in a war fighting operation (Gulf War, Afghanistan) for at least one month (or are wounded during the qualifying period), a scarlet surround.
You would only get one trades badge so altered - if you did two tours in Cyprus then fought in Afghanistan, you would get the scarlet edged badge. The tours in Cyprus are already noted by your campaign medals.
The trades badge would be for the trade the soldier was employed in during these duties.
What this would signal, then, is employment as a, say, infantryman in a hostile enviroment.
At the risk of looking like the boyscouts, if you changed trades after serving in an infantry company in combat, and say remustered to supply tech. You would keep the unpiped supply tech badge in the position normally worn as a "current trade" and on the opposite arm, you would get to keep the piped badge showing the trade you "saw action" in.
You would only be entitled to two badges maximum - one to reflect your current trade, and one to reflect past peacekeeping or combat service.
This would only be useful for ranks of Private to Sergeant, but perhaps WO and up could wear a miniature of the piped badge above the ribbon bar?
What this does is show at a glance that a guy has been in combat, or actual operations, as an infantryman, say. Currently, a soldier wearing the infantry trades badge and the UNPROFOR medal could likely have earned his medal as a clerk in Italy, or a cook in a Yugoslavian headquarters, and remustered to the infantry afterwards.
Just a dumb suggestion, but surely something could be done. The Americans had a big problem with introducing the Combat Infantry Badge because initially the combat medics -attached directly to rifle companies in WW II - were not eligible for them, despite putting their lives at risk as much as, or more, than the infantrymen.
There is still the problem of criteria, I realize - ie how do you quantify how much a person has been shot at, etc., but I don‘t suppose that matters much. The difference between the infantryman in Afghanistan - carrying his rucksack up and down the hills - and the seaman doing equally important work for 8 hours a day in a climate controlled ship - isn‘t recognized currently by the medal or even the trades badge.
Thrown out for discussion knowning full well some may hate it. That‘s ok, but let‘s hear some other suggestions in that case.
