For plugins, maybe we can move all everything from directly interacting with m_MixPlugins to an interface class that lives in PlayState, and either redirect the calls to the individual plugins (regular playback) or otherwise record parameter changes like we do with m_midiMacroEvaluationResults in PlayState already.

Currently a pattern delay + pattern loop on the same row cause an incorrect song length calculation (see https://www.un4seen.com/forum/?topic=15448.msg143803#msg143803). I don't see an easy way to implement this correctly with the current implementation.

Another obstacle here is that all the playback code currently relies on calling Order() instead of Order(SEQUENCEINDEX), i.e. it depends on the current sequence chosen for editing. I have a working copy where I intend to phase out this feature to allow for multiple sequences being edited at the same time in separate windows. I'm not sure if full multi-sqeuence editing needs to be implemented as a prerequisite, but it will definitely require some modifications to the playback code.

RowVisitor::ModuleTooComplex also needs to be able to be turned on dynamically for regular playback code.

Seeking could be optimized by not considering NNA channels (GetNNAChannel should not be called or always return CHANNELINDEX_INVALID), channel loop in Sndmix.cpp should only go up to pattern channel count.

By allowing to override plugin handling in PlayState, a more elegant solution to handling PC events -> SetParameter in mod2midi would be possible.