Chester Rabbit said:
Deus Ex The Conspiracy. I desperately wanted to play Human Revolution but everyone kept talking so much about how great The Conspiracy was and how Human Revolution still didn't measure up.(Though it was a step in the right direction)...
Oddly, I haven't heard of Conspiracy.
I can't quite think of deliberate predecessor-hunting that I've done. What I will do instead is turn the OP's question on its side and address it that way.
I did play the original Deus Ex within the last two years. However, recently, when I finally sat down with Human Revolution, something bothered me about it. Oh, certainly there were a great many map and cosmetic similarities to DE (names, fate-of-mankind issues, fire escapes, tiny exterior maps, hand movements while running), and these hallmarks were well chosen. Yet it was the fundamental gameplay that bothered me. Why does Adam shift around uncomfortably when speaking? User selected dialogue responses... well, OK? And then when I climbed on a ladder for the first time, I realised what the problem was. I was playing Alpha Protocol: Human Revolution. And if anyone, anywhere, had told me that it was Alpha Protocol in JC Denton's clothing, I would have saved my money - for a slightly better Steam sale. Not that Alpha Protocol is a bad game; it's just that it does so many annoying things (like unnecessary, but obnoxiously difficult boss fights), and that those annoyances really wreck the replay value. [Sorry. I'm still annoyed, and the irritation has leaked out onto this page.]
Having played Bioshock, I'm keen to play its predecessor; System Shock. That game has always been a faint blip on my Radar.
Having first played Fallout 3, I went on to play The Elder Scrolls IV: Oblivion. Morrowind is on the to-do list and already in my Steam library. (What? It is too a prequel/sequel!)
After purchasing the big iD bundle, I played through Wolfenstein 3-D again. Then I started to play Hexxen - and I just couldn't bring myself to complete even the first level.
Wait, now I remember. I played the demo for Amnesia:tDD. When I bought the full game, I made sure to play the Penumbra games first. It wasn't necessary, but very interesting.
And I bought/played Tropico 3 after playing Tropico 4 on a Steam weekend. Tropico 3 was a better deal, as T4 has interminable DLC and is a dressed-up T3 that is essentially the same game. However, I do appreciate some small improvements that were made in T4.
Additionally, I try to mention this when appropriate. There is a prequel to DOOM3 and Half-Life 2 called 'They Hunger'. OK, its not exactly a prequel, and it feels rough around the edges, but it introduces some game mechanics and scenarios that simply were not in Half-Life, or DOOM, or DOOM II, but are evident in the last iterations.