Honestly, I'm kind of against remakes and remasters. While I admit that there are a few great games that haven't stood the test of time, the majority of them are fine just the way they are.
Remakes don't always have to be about revitalizing games that wouldn't gel with modern audiences. They serve a number of purposes. In some cases, such as with Pokemon, they're used to bring the still playable older games up to speed with the various mechanics that would later be introduced. Other times, they can be used so that the creators can take a different direction with the game, generally one they either wanted to begin with or is simply an alternative to the current take they didn't much care for.
Hell, even remasters and...HD remakes (...which are just ports, generally, or are only remakes in a technical sense but not in one of substance) have their uses. In some cases, such as Shadow of the Colossus, they're used to bring games that were too strong for their original consoles up to speed. Another use, which I would love to see with .Hack G.U., is when the games are expensive as hell and are hard to obtain. These releases can revive popularity, make the games more accessible, and sharpen them up all at once (generally at a pretty good price).
So it's less about whether it's necessary and more about whether what comes out is worth the investment, because regardless of what happens, the older product still exists, and as the consumer you really aren't affected by the release of a remake/remaster.
*cough*
As for my remake choice, .Hack. The original. There's a lot I would love to see added to G.U., but I worry that more would be lost in the redubbing of the game's voices (which, from Reminisce onward, are pretty damn good) than would be with just a remake. Plus, if anything, P4's proven just how much a port can expand on the original work. But with the original, it lacks a lot of the style and flair that CC2 is known for, and as far as gameplay is concerned, it has a lot it can learn from G.U. and Link (largely in the team management and order department). Its open level design and dungeons could also be expanded for more varied exploration, making each combination of key words its own adventure.
Oh, and multiple characters- or hell, even multiplayer. G.U. remedied this by creating the Adept Rogue, which allowed you to switch between weapon types to bring more varied play. There's very little that would need to be done- in either game, really, to make it so that you can play as other characters since they all have developed movesets and operate similar to the PC.
So yeh, .Hack remake and G.U. remaster/port are my major picks. Oh, and Soul Nomad. That game's expensive as hell and never got an updated PSP port like its brothers. C'mon, NIS; that game's fantastic. It doesn't even have a PSN release.