Aside from better graphical effects (ability to zoom, bloom effects, HDR, weather effects), I don't think the interface of Syndicate needs much more work; it is as intuitive now as it was back then. Obviously we'd also have better physics (destructible buildings, etc) and improved AI (pathfinding, NPC intelligence, etc). What I think could really be enhanced is the customisation aspect of the game.
Syndicate was brilliant in that you could research new technologies (with new discoveries in the field speeding up your research path); these technologies were either new weapons or upgrades for your cyborg agents. This was all well and good but eventually you'd have nowhere else to go and customisation stopped outside of the ability to equip your agents for specific missions. What I'd like to see is the ability to research different technologies (bio weapons, stealth, WMD, etc). Within these broad categories the game should allow you to them create your 'own' weapons configurations, for example trip mines that release nerve gas, trip mines that release paralysis gas, trip mines that set off a large explosion of napalm, etc. Instead of plain rocket launchers (GUASS GUN!) you could fire different projectiles (different gases, napalm, etc); instead of flame throwrs you could have short range gas propellants (nerve gas, persuasion gas, whatever). The point is that you should be able to develop the 'base' techologies and then start to create your own combinations of them (having say a primary and secondary fire ability).
Instead of just high-level control over 'taxes' you should be able to manage your territories on a lower level. For example, you should be able to set how your police force acts (Brutal or passive), how your citizens live (poverty or luxury), how much propaganda is forced upon your citizens (very little, extreme) and how much choice people have (full freedom, facist regieme). Some of these would cost money (eg: lifestyle), some could cost you money (choice options) but all would ultimatley affect how your cities work. You could choose to create an air of happiness wherein your citizens are treated well and have a full life choice or you could be totally oppressive and ensure that people are brainwashed on a regular basis).
Why? The reason is simple and obvious; it will let you play the game in the way you want. Not just play the game, but run your company. The overal goal in Syndicate was to rule the world with your corporation; the goal would be the same here but the emphasis would shift away from just ploughing through the missions with guns and winning. The choices you make would directly affect the way the world works around you and the way you play the game.
An example; imagine you were trying to make a move into a territory wherein the current populous were overjoyed with their current government. Your mission was to assassinate the current governor and take over the territory. You'd have to a) go in with all guns blazing, b) sneak around and try and blend in with the population to acheive your goal or c) not acheive the mission, but employ some persuasion and/or propaganda technology to weaken the grip on the current city. Option c) would not be a failure, you just wouldn't take control at that time but it would potentially open up the city for an attempt later on. Option a) could prove unsuccessful as the populous would actively stand up against your insurgence unless you took extreme measures (eg: genocide). Option b would require covert technologies and a specific strategy, perhaps you'd spend one 'mission' scoping out the city and them coming back more prepared. The way you attempt each mission would have an effect on how the population saw your company; if you chose to murder everyone in your way the people of the city would hate you and you'd have a very low popularity rating when you're in control (perhaps meaning rebellions and open doors for insurgent attacks). If you chose to undermine the popularity of the governor, people would be more responsive of you in the future - potentially you'd be seen as a 'liberator' and be welcomed.
It is THIS sort of element that Syndicate should be updated with. The game would then be completely open to new ideas of players - you'd be in the hotseat.
What do you think?
What I'd like to see is an interface which relies more on the keyboard; nothing is faster than the keyboard for switching agents, moving them around and giving them complex orders. I also found it difficult to move up and down ramps, and correctly drive the cars. Although, I was playing it on the Mac, which was already awkward as I had to use SPACEBAR to imitate right-click.
You'll note that Syndicate already had various values for the government (fascist, etc) and the level of control over the various governments between missions. This indirectly affected the difficulty level of each mission's AI "cheating".
Syndicate absolutely does need more of an advantage for more casual players. I'm pretty hardcore and even I was taking serious amounts of damage in almost every mission, often ending in disaster when choosing cover poorly (what the fuck do you mean those red bastards can shoot through windows?!?)
Research and bionics also needs to be updated to fit with modern times; myoelectric limbs are so yesterday compared to aggressive biotechnology and custom-rigged neural implants.
Also, the mid-mission interface needs some work. Syndicate's equipment/research screens are rather complex (a limitation of the low resolution) and would definitely need to have at least tooltips or some overall 'strategic advisor' functionality. There's also a lack of feedback from your changes; there need to be more immediate effects or possibly a random event or two that you can take opportunity on. Personally, I would even add a system of shifting alliances; buy out your rivals while destroying their facilities and soldiers.
These are all nitpickings; note that the game is utterly brilliant. There's nothing like opening up four agents with full-bore miniguns and chewing up hostile agents in a crossfire while trying to pop a scientist.
Syndicate is a brilliant, complex, but flawed game. Luckily for Syndicate, almost all of the flaws are in the "big game" (world domination) instead of the "small game" (hit squads). Revising the "big game" to be more accessible and yet also with a depth of choice would be a powerful change, and one that would make the unbalanced gameplay of Syndicate far more effective.