I happen to like the TS3 aesthetic as it is now. If it was changed to resemble TS 4 I simply would not be interested in the remaster/remake.
A while ago I hosted a long thread in the TS3 section and the question of remastering/remaking came up. There are a number of difficulties which would need to be overcome for it to happen including making a 64 bit version. Essentially the entire programme would have to be rewritten. Granted this would allow you to turn TS3 into whatever you wanted but any time you do major rewrites you risk creating new glitches and bugs.
Assuming the difficulties were overcome I would love to see the major glitches and bugs dealt with, particularly in IP where the routing is a real headache without modding. I would also love to see inside the rabbit holes and to watch my sims driving their cars around from the inside of the car. Other than that I would like to see proper babies with legs. Most of all I would expect the colourwheel, open world, swimming in open water, diving, world editor, CAW, and terrain tools to be retained and enhanced where appropriate.
That is just for starters :) If there were to be no new iterations of the franchise then I would like to feel I had a game I could continue to play for another 11 years.