Just adding to this thread really:-
The news that now SSL Native plugins, at the release of v6.2 are supporting this functionality for Pro Tools (which will no doubt soon migrate to Studio One, etc…). From their website:-
(SSL Native) V6.2 also includes the addition of Pro Tools gain reduction metering for the Bus Compressor, Channel Strip, X-Comp, X-Valvecomp, Vocalstrip and Drumstrip plug-ins…
+1
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And option for VU Meters too (like PT) 
VU meter/Peak meter/both option for all audio channels - Cubase - Steinberg Forums (Cubase 9 thread).
In addition to GR Meters, Pro Tools has 17(!) metering options for the mixer channels:
Time for a bump - Nuendo folk would love them too no doubt… (plus all @Starsprinklers options.!)
Moving forward. Pro features. Increasing value for money.
I think this can be generalised a lot. Gain reduction is of course one of the most important items to get in. But it would be great to a a generic mini-ui that fits in to mixrack and could jack in to control parameters in some form a standardised set. They mini-ui may not even need to be rendered by the plugin, only data driven with some small gui-objects like a level-meter, db-value, knob and button.
+1000