Anymore the Cage match is only used as a tool. You mainly only see guys in it, that can do something off the top of it. (ie. Jeff Hardy) If its not used for that, then its used for something completely ******ed and unworth watching. (ie. McMahon/Hornswoggle) Its because of those two things, that the Cage match has lost its mystique.
If you're asking what more can be done inside a Steel Cage, to make something never seen happen.. it reminds me of the Summerslam 2001 match with Undertaker and Kane v. Diamond Dallas Page and Kanyon, when Shane McMahon and D.D.P. were suggesting Undertaker deliver a Last Ride to D.D.P. from off the top of the cage.. however Taker was against it, in fear that it could seriously hurt him, even fatally.
Now, in all honesty.. I think if that would've been done, it would've been forever remembered as one of the most unbelieveable sights in a cage, ever. Similar to Elix Skipper's tight-rope walk, into a hurricanrana.
The thing about why it'd be hard to ever have another absolute classic is as you've said. The Cage is used almost once every month. Its not exactly that its become "another match" but its no longer considered to be a special match. In the fact that it happens so often anymore.
The other thing is, in order to have a classic cage match, you truly have to have the right type of feud, individuals, and overall setting. You can't just throw two people in. You can't just have it randomly appear in a feud. It has to have a purpose, a reason.
Edge v. Matt Hardy - Unforgiven 2005, that was a Cage match with purpose. It wasn't to settle a feud, but it was to contain it. The previous p.p.v. match between the two was stopped due to "blood loss" so therefore, the Cage was used to not only "hold" Edge from escaping.. but also keep the match from ending without a true winner.