Connect:Amp as LFE sub only output device.

LFE? As in a 5.1 or higher audio/video signal? Or just straight music?

LFE meaning "Low Frequency Effect" which is what the " x.1"  represents in any 2.1,5.1,7.1, etc surround setup. 

The only configuration utilizing a Connect:Amp currently possible is 2.1 (and possibly 4.1 if using a second C:A setup for rear surround, I'll have to test that.)

Sonos does not have a physical LFE output with any of their soundbar products. 

My thought is Sonos could allow the configuration of a single Connect:Amp as just the LFE output ( not using the two speaker connections) in conjunction with a soundbar. While using another C:A to power the rear surrounds, possibly even setup a third C:A for side surrounds in a 7.x setup. 

They likely won't do this, but I think with programming and software updates it could be possible. 

The lack of 3rd party powered subwoofers is one of the major limiting factors when using Sonos products in a theater setup. True audiophiles will use something other than Sonos products anyway, but for those of us who already have multiple Sonos devices this would be a fantastic feature. 

I’d think you could use the LFE out by itself, even on a plain Connect.

Grouping it to other Sonos would likely “work” for music sources, the delay would be an issue for TV sources. They would of course not be aware of the sub or have their response curve adjusted.

Matching levels at volume changes would be an aggravation.

I’d either commit to the soundbar and Sonos Sub or stick with passive speakers on your Connect Amp. You could go to the new Amp if you wanted HDMI TV sound input.

Sonos does not offer subs to match what I already have. 

Using the line-in input directly from a TV to the C:A in a 2.1 setup works great as far as A/V sync timing. I'm using this as a small setup in a secondary system in the home with a pair of B&W monitors and a Velodyne DD10+.

I liked how well it worked so much that I entertained the thought of adding a sound bar for a 5.1 arrangement. 

Sonos is making clear they don't have an interest in supporting higher end setups, rather selling more of their own ecosystem hardware. 

How difficult would it be to add at least an LFE output on a soundbar? Many other manufacturers do it.

Based on my research they could even enable LFE using existing hardware with C:A products. 

Sonos isn’t looking for new projects, they are struggling to fix the problems the May updates caused so don’t expect anything to happen soon even if they accept the suggestion.

Another reason to avoid Sonos products. 

You'd prefer they don't get current users back to where they were first, and instead go do new stuff?

 Now for me that would be a reason to be unhappy with Sonos.