I personally would rather see any efforts put toward actually coming up with new content.
You mean all existing content should be integrated in TBP. I doubt that anybody at the moment is working on new stuff, or is there?
Any new campaigns should be put out as addons to TBP.
I don't know how this would even be handled and would rather like to have everything inside of the main game. Also see above...
I am working on a campaign, currently I am in a phase which involves stuff that is technical minutae to non-FREDers (I was able to change to tracks to work on another project because some of the execution is based on shared principles - its very synergetic). The nature and lenght of the campaign means its also hard to really show stuff, not mention that a lot will be filled placeholder graphics.
Integrating all content is going to be difficult going forward. Some of the fancier systems acutally rely on addon tables and having multiple of those are going to cause conflicts, and it is not always practical to intregrate those into the main install. A prime example is iff_defs.tbl which handles the "teams" during a mission (e.g. Friendly, Hostile, Non-Combatant). The limit on possible IFFs is 8, and it is a very useful category for mission design as you can run distance checks and arrival anchors of e.g. <any hostile>.
However, making a seperate release on Knossos that uses TBP as a dependency is really easy. It's two easy to do steps which are required part of making new mod on Knossos anyway: 1) Inputting that the Mod is dependency of TBP and 2) setting a compatible version of TBP for the mod you are creating. After that you can go hogwild in your own little corner without worry if you are going to break the main TBP install.
re: Training Messages
To be fair, the Training Messages were never actually meant to be used that way they are used in many old campaigns - they were meant for the tutorial missions only. But with limited ressources they were frequently approriated to display stage directions or hints. Today, you would either use the show-subtitle-text SEXP or a custom HUD gauge - but both were either underdeveloped or non-existent back then. (I think we had the old, hard-to-handle show-subtitle SEXP back when 3.4b released)
I remember that I did move them for my own work, but until I have the custom Detector and Timer gauges up and running for TBP, I don't have any results to share. I moved them unter the reticle and above the radar.