Concerning RA1 and older, I would love to see updated graphics, but of course faithful to their designs like Chad1233 said, and an updated UI that is easier to go about using than the two-column scroller. The most sacred thing I could say that shouldn't be changed are the base concepts of units and factions. I think that was where I felt RA3 fell flat since it tried to change up everything a bit too much.
As for RA2, if it were possible to overhaul the game entirely with a new look, but the same layout, gameplay, and faction styles, I would be all for it. And if Generals were to be remastered, I could definitely see it looking like the cancelled Generals ( unfortunate ), with graphics and gameplay worthy of a 2018+ RTS.