What would it be? Let's pretend there's no temporal distortions, and you didn't go back and actually ruin the game, so that the game you wanted to go back and develope is a sucky game now, and you never would have chose it in the first place, or would that be the second case? 

Anyways what mmo & why?

Whats the point of getting back in time if I cant cause temporal distortions ?

If I could, I would have forced Sigil to get a damn real businessman and a damn real lawyer into their leadership section, who would make ironclad contracts with Microsoft so they cant weasel out of them and cause Vanguard to be released far too early, far from finished, and taken over by SOE very soon, while Sigil went bankrupt.

Pretty much the same for Troika Games, by the way.

I would go back in time to fight tooth and nail to keep Bioware as an independent developer. Nothing more.

Remember when Bioware was still independent? Those times were good.

Ignoring what the OP said about temporal distortions...

I would go work on SWG, that way I can hang out with the amazing Raph Koster.  Also I would sieze all sleep and hobbies and do nothing but work on SWG for a year straight, leaving it without bugs at launch so that it gets better reviews and WoW copies it and today would be an amazing assortment of wonderful sandbox mmo's and I would not be here posting on this thread because I would be enjoying all the wonderfulness.

SHADOWBANE

The greatest pvp game ever conceived !

Grand Chase and bring it back to season 2

Star Trek Online.

It could have been SO much better. It's improved in the last while, but it's still nothing like what I was hoping for.

- Way more tactical starship combat, kind of like EVE, but not *that* deep.

- Improved "away team" missions where there are lot more options for diplomacy, etc.

- WAY more polish. 

Also, SWTOR would have been basically SWG 2 with space combat akin to X-Wing vs. TIE Fighter.

Warhammer Online

I'd try to kick behinds here and there to keep things on track and maybe save that game :P

Definitely World of Warcraft. I would have stopped the madness that led to todays ever increasing simplification and cutting of content in the game.

Right at the first expansion stop questing from becoming chain grinding. Stop the separation of pve and pvp by gear. Stop the destruction of the game lore, e.g. no paladins for horde and no shamans for alliance. Stop the oversimplification of raids and battlegrounds. Most of all, actually add more content to the game.

SWG

I would kick Koster out on his backside and make that game with actual worlds instead of random generated crap that had a ditch every 20 steps.

There would have been an actual 3D X/Y axis allowing you to jump, not be running or on a mount and get stopped by a pebble...and you wouldnt be able to shoot through a hill to something on the other side of it.

The random quests wouldnt be spawning in housing areas so no creature mounds would be spawning in someones house not allowing them to access it unitl a server reset, let alone the person not being able to complete the quest...

The gathering system would have been done without the randomness, or the ability to place a structure to do it for you. That system favored those that didnt have to work or go to school at the times of the resets leaving less for everyone else...and it promoted hording.

and there would have been random battle events between the empire and rebels.

Oo I love hindsight threads.

Here's what I'd do before traveling back to the past.  I would find a winning lottery number for a huge payout, then play that number in the year 2006.  Next, I would stop Bioware from even thinking about EA, and instead become their top investor.  With enough investing, I wind up becoming a producer.  SW:TOR would have its own game engine and things like space gameplay would mirror a game like Earth & Beyond.

One can dream.  Short answer: Win lottery, fund SW:TOR.

Aion

I loved the concept but the execution was not what I had in mind.

SWG and stop SOE killing the game...