I'm not sure we'll see another after the 8600, but its continued enhancement through firmware is hopefully going to continue. In the early stages I seem to recall Eric Cottrell dismantling his and finding some potential for hardware additions. I'd have to search back many years but it was posted here at RR.
Some simple things could take it a long way, IMHO, such as a breakout for an external monitor. This alone would be an incredible addition to it. Whether or not it's possible I have no idea, but about seven years in it hasn't happened so I would not hold my breath. Obviously it can be used with SDR based software and I do that if I use it on HF by using SDRConsole. It works well with WFView also but I've been consistently unhappy with the display quality in full screen mode. WFView seems to be the only PC based software that retains access to the basic hardware controls of the receiver. SDRControl is a great iPad or Mac based software as well that does the same. The breakout to big screen viewing would have been the best as far as I'm concerned.
We know DMR ain't gonna happen, but there is an easy workaround for that. Instant decode of CTCSS/DCS would be nifty. Hasn't happened either and it is easily overcome in the same manner monitoring DMR via DSD+ is.
It's pretty well documented at this point that I've had a love/hate relationship with this radio. I've bought 'em, I've sold 'em. The guy above this post loves to chastise me for this, so he'll be thrilled to know that I've had another very quietly mounted center-console in a Novexcomm 3U mount for several months now. I can't stay away from the damned things.
As mentioned, I think the future gets smaller. I have several Airspys. I have the new SDRPlay nRSPST networked SDR which I am slowly but surely coming to greatly appreciate for its easy remote access capabilities. I just ordered one of the RX888's and I'm going to try to set it up with OpenWebRX+ at some point if I can figure out how to make that work. The Airspys and SDRPlay devices I think are where the future of wideband receivers are, hell, they've been the present for quite some time now.
The 8600 exists in such a niche market, I think as you mention Marcy it's quite possible/likely that this is indeed it, in this particular form factor anyway, but the tech out there is amazing and we shall see as time passes.