I see the game engine being a good basis for revamping. V feels like a facebook version of IV, very simple and bubbly UI and constantly spamming achievements at me
Maybe I'm just teching way too fast. At 150 BC on Prince I've basically slingshotted my way to the Renaissance era, dumping my Culture points into a completed Rationalism tree. This gave me the means to unlock Cavalry just a little after 1AD and proceed to take out out 3 civs with only 4 of em (Medic is ridiculously overpowered now, basically medic III from a tier-2 standard unit promo).
Unit mechanics... if there's anything well done, it's the combat system. Combat odds are much less mathy and roulette odds. Units can attack without having a 15% chance to suffer a heart attack during battle (I've always considered 85% odds to be good in IV). The tile occupation/collision is more annoying than strategic though. Extra roads are out of the question, and chokepoints can simply be Operation-Human-Shield'd. But building and moving an army across a continent is like trying to process a caravan of gypsies through the DMV. Last time I checked roads could be built with two directions of travel..
Siege units are treasured behemoths since they can safely attack and level up like everything else, with support. Usually this just means having a melee unit between the hex you're bombarding. City defense is a nice little addition, I suppose.
ergo..
I hope somebody will mod the following changes:
-Decrease the production costs for most city buildings. Wonders and unit costs are just fine. If building military got any cheaper, or more (insert stupid new

art, somebody please redo the icons back to

)) were available it would make domination even easier. In fact I can't wait until somebody overhauls the icon art...
-Redo the gold rush-buy-from-turn-1 crap. I want to believe that the intention was just to give the player another victory path or means of control, but I would much rather have more appealing ways of winning over city states. Buying land tiles isn't such a bad thing, but units and buildings feels off in the BCs. Slavery, Drafting and Rush-buying came at appropriate times in IV.
-City states would be be recoded to gain favor in more interesting ways. Dispatching little bands of bandits when I'm opposite the map, or begging to kill the other one isn't my idea of fun. Searching for a
natural wonder was an interesting one I got, I feel like that's a good start. Still, right now it seems like they're just easy targets to invade that usually contain needed luxuries. I feel like earning their affection could be lucrative and fun for the game with the Patronage policies and having a lot of friends, but I haven't tried it.
I would rather be able to spend culture points to permanently (or as permanent as any other establishment in the game is) achieve allied status etc., or have to make favor-influencing decisions. Maritime could favor high

or

overflow, and military could favor high troop count or consume a GG for permanent alliance.
-Fix the awful nested production Queue GUI and bring back the alt/shift/ctrl shortcuts! Just bring ALL of the shortcuts back.
-Give us our information back for god sake! We need mouseover tooltips for every tile and unit. Having to click on everything is very 1995. The game needs an "advanced" option to have things like citizen assignment and queue management automation removed. I.e. the game will start in manual config mode always.
-Workers do so little for the game now. Aside from improving the much-needed luxuries, the rest of their contribution feels quite moot. This could be easily fixed by having some later techs (not just social policies) make improvements like farms, trading posts, and mines grant more resources from being worked. Buildings like the market could expand on luxury or strategic resources much more than they currently do. The seaport, for instance, takes anywhere from 70-30 turns (with two seafood) to make up for the hammer investment. But I digress...
-Food resources are inconsequential aside from the meager surplus. Not that managing health was a big deal in IV, but at least you could trade food resources. I would allow these to be traded or gifted to City States for favor.