I personally have played all parts of the series (and its brother System Shock), but found myself putting most of my time into the first deus ex, finding different ways to solve a lvl the 3rd time i finished it... got it as a ps2 port way back and silly controls aside, was hooked. Buddy was forced to watch in awe and then seek his revenge in tekken later...
I enjoyed Human Revolution, but after I made a savegame near the end and pushed each button once, I was pretty much done with it for then. I finished some levels of it by throwing drink dispsensers around and others by turning a sentry mobile, which was fun...
Maybe HR would have been more fun (also my prediction for the next title if they dare to dare) if it had a hint of online interaction, ala dark/daemon souls. Hell, imagine a mp variation of the game where players race to steal the data from a (very complex) complex or cooperate ala that one sam fisher title that had awesome coop.
Hell just make a good editor for HR, add a easy server/download system that looks like a map offering "missions" from the locations the server is at and add the top 3 usermade maps (every week) to the offical server map rotation, you can even glue on a lvl up/upgrade system ala modern warfare, based on what you do. Super sneeky, super sielent? Get that placeable cloaking field. Utrist McHackalot? Extra EMPs for cheaper, less dmg taken from any robots/turrets...
Now imagine, you come into the lobby with your friends, you get the mission description of one of the many new maps that pop up every week "Agents, you need to find... bla bla" - and then you start to loadout. You have a certain amount of credits, which can be used on equipment or upgrades (some cheaper for some specialist) - you know one of your buddys is a combat fella, another maybe a sniper - if its wellmade, you can even see there loadouts - and then you go in.
Unlike singleplayer, the levels should not be stuffed with items, but have some logically placed caches of weapons and equipment that you can, but don't have to access/find. Causing alarms makes it fairly hard and default difficulty should be quite high, encouraging teamplay. Bonus xp and therelike if you manage to go silent/nondeadly