A lot of plugins out there unfortunately do not come with makeup gain built in - including a few native S1 plugins (Chorus, for example), even though they do affect the volume..
I often use a mixtool after these plugins to perform the makeup-gain instead, but then it becomes difficult to A/B the plugin reliably because it's tricky to bypass both the original plugin and the mixtool plugin simultaneously so you can effectively A/B the plugin at a matched volume. We all know that if it's louder it automatically sounds better, which can trick the ears, so volume matching after every plugin move is something that I think should be a lot easier in S1.
I think it should be possible to associate any instance of mixtool with any other plugin in the chain, so that when that plugin is bypassed, the mixtool is also bypassed, etc. It would be a bonus if mixtool also had automatic makeup gain for any plugin you associate with it.