As others have said, sacred are the soundtrack (or rather sound in general, the death screams are perfect) and FMVs. Except quality of life improvements and other "new" standards (such as production queues, air units regular movement control, veterancy ranks... stuff already present in later C&C installments), the gameplay is sacred as well -do NOT add secondary skills etc.
I wonder whether they are going 3D (kind of what at some point they said Generals 2 would evolve into, a single engine for different games) or stay with the same design with upgraded sprites (if they achieve the quality of TS and RA2, that would be perfect). In any case, don't change the the way things look; I fear EA might be tempted to redo the artworks for the sake of artistic direction consistency with the later games.