Have you played the Pinball Circus though? It looks like it would be fun, until you get the ball to the top. Once that happens, there really isn't any more to it at all. I played it a number of years ago when it was at Expo, and I got the ball to the top on my first game. That was more than enough to me.
The extreme cost to do something like that (so much metal in that game) combined with the fact that I don't know how you could make the game deeper kind of doomed that one. Not to say it couldn't work, just not like that.
Quoted from LTG:Safe Cracker sized cabinet and board game in head. More were planned but never got past Safe Cracker.
Sega did get Mini-Viper to test too. Wasn't that one first, actually? I know it didn't have the board game in the head, but I thought it actually played better than Safe Cracker did.
Quoted from mechslave:P2k was a failure. It did not earn. RFM's were puked back out at auctions within the year for half price. South Park crushed it in earnings. SWE1 also outearned RFM, but it wasn't enough with the price hikes. They put all their eggs in the P2k basket, but the flaws caught up to it.
South Park earned more, but the P2K machines held their own. RFMs popped out quickly because between Williams closing the pinball doors and the system not being completely reliable, a lot of ops tried to get their money back on them quickly, foreseeing them as a bad long term investment with such little to repair, from what I've heard. SWE1s tended to stick around a lot longer, as they earned really well as you mentioned. There is still at least one on route around here.
For innovations, I'll say blinders like on Tommy.