@256 yeah these games bottled out and even with better graphics actually didn't need to advance too much (versus the costs - just like a skins pack for all the rest of the games) or the others are good but this is what shoulder sit on (literally as the Engine I suppose that's quite true!)
If I missed anything new about these games do let me know but I'm even thinking is there more (Halo was major one-up in 2000's for it's mixed story, atmosphere, fucking scary moments, co-opt story mode, vehicles, multi-player, enemy noises, battlesuit / temporary shield, computer narration / Cortana? / maps etc etc) and then more of the same...
Some western games impressed me for it's theme, choosing style of 'slinger' you are, the atmospheric level settings, vocal narration like a ral western film and objective-based play... Call of Juares
https://yewtu.be/search?q=Juares+game