It took me much more time to make my opinion about Remaster of CnC than i expected. I guess it should be more subjectively, cause of impressions. So for examples I'll take my most favourite CnC - Tiberian Sun.
So, first thing, which i want developers to pay attention is old CnC games details which made such unforgattable atmosphere. Old CnC games had charismatic faction design. Stylish black and red gamma with unusual geometry of NOD faction, which units, buildings, UI elements, decorations in cutscenes and, off course, Kane - all of these things show us NOD ideology, their mystical origins and what it really powerful faction is. Not only by what we can hear in briefings and read in descriptions, but cause of these details. In other side GDI - now it can be called as retro futurism style faction, with heavy armored things, which were little bit banal looked, but still charming. Both of these factions has their own fans. So as enviroment, which can tell us a story through images. Tiberium infected nature, wasted cities, darkness, ambient soundtrack. These details makes the atmosphere, and I think it's most important to keep it. CnC 3 is a little weaker then TS in this case, but it is still looking like Tiberium series game. CnC 4 lost all of these cause of cartonish and banal design ( and, surprisly there were almost no Tiberium and its effects, it's hard to link it to other games )
Second thing - gameplay. And before i'll explain my opinion of this, I'd like to say - there is should be full remaster of any old game, but it should be not CnC 4 like experiments. People there said, I guess, lots of things about how gameplay should look like ( honestly I didn't read all posts before ), but, I'll say that it should be freshed, actual gameplay in CnC borders. People argued at CnC 2013, for example, because their developers tried to make Starcraft, not CnC. That was really madish weird.
And the last thing - in CnC there is lots of 3D rendered short cutscenes with units. Remastering these cutscenes, making things which can remind these references - it would be best way to wake nostalgia.
Good Luck with remastering, and ressurecting Command & Conquer!