You have defined your technical goals for this engine, but that's not enough. You're not just making an engine - you're (re)making an entire game, and unless you're recreating the original verbatim, that requires good game design.

In this case, since you have a set of rules and know that you will be deviating from them in the remake, this essentially means that you need to define what direction and to what degree they will deviate.

In your wishes thread I saw a lot of suggestions which are great design ideas in themselves, but which would detract from or destroy the feel of the original game.

For instance, targetable subsystems in units would complicate things immensely (just an example - the suggestion was more or less rejected) and first-person controlling of units is being talked about.

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Tiberian Sun is a good game that is playable, enjoyable and could easily be called 'complete'. The reason you want to remake it is (I assume) because of the limitations of the original engine as far as modifying the game is concerned.

Therefore, I would like to suggest that you don't worry about redesigning the game, but rather do recreate the game verbatim - but only where the specific rules are concerned.

The rules I'm talking about are of course the unit statistics and game mechanics - just like the abilities of different chess pieces (and the end goal of course) essentially define all the 'rules' in chess, so the myriad details like cost, health, speed, etc. define the 'rules' for TS.

Don't change them. Instead, remake them exactly, but redesign the engine itself from the ground up. Add all the things that were missing from the original (unlimited armor types, etc) and make the game completely customizable - all soft coding, wherever any rules are concerned. This will fulfill your ultimate goal, which is to allow playing around with TS and making it the game it could have been.

This means adding the ability to do anything you think could enhance the game - like first person unit control. Incidentally, Renegade was made using Westwood's SAGE engine, which is of course their 3D RTS engine; first person unit control is essentially what they did. But it doesn't mean that you have to incorporate it into the game yourself - just add the capability to the engine and let modders handle implementing it.

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Forgive me if this seems like an obvious goal, but it would be very easy to turn this from a project to make a more easily and thoroughly moddable, graphically updated TS into a project to make an entirely different, TS-derived game - essentially a mod in itself, albeit with a new engine.

Of course, this is a perfectly acceptable goal - if that's what you want to do with it...