The one thing I don't get is why people say that games in the same Gen take place at the same time. I've never heard anyone offer up evidence as to why FR/LG and R/S/E or DPPt and G/S/C/HG/SS take place at the same time. They just go with "oh, they're in the same gen, so they must take place at the same time".
DPP opens with a news report on a red Gyarados appearing at the Lake of Rage.
HGSS has an event where Erika tells Jasmine that she thinks Jasmine would like the Contests in Sinnoh and should go participate. Jasmine appears to give the player Waterfall in DPP and also appears as a Contest opponent.
There is a fair amount of evidence pointing to gen 2 happening at the same time as gen 4.
If this is the case, then Emerald doesnt take place with Ruby and Sapphire! The problem is that R,S, and E are simultaneously parallel to each other in terms of time. Now, I am not really that sure if Gen I and Gen III took place at the same time, we don't know. The only thing I could think up to tie FRLG to RSE is the Sevii Islands sidequest, but it could aswell be a time capsule for all we know.
I know the timeline is whack as hell but I'm guessing that GF will probably pull off some retconning in ORAS like they did in RSE when the Steel and Dark types were introduced whereas FRLG was = to RSE in terms of time.
Emerald's place in the timeline compared to RS is most similar to BW's predicament. It's basically an alternate universe, an alternate scenario. Obviously Black and White can't happen in the same universe, just like Emerald can't take place in the same universe as Ruby, and Ruby can't take place in the same universe as Sapphire. Their place in the grand timeline would still be the same, though.
Retconning will happen, but there is a big difference between retconning gameplay mechanics, and retconning actual plot.
There is no Time Line. There never was.
Giovani was at PWT because he is cool. No other reason.
Excepting the Driftveil Tournament, the PWT is not canon. There is no way Caitlin could have aged from DPP to BW/B2W2, but Bugsy did not age from HGSS to B2W2.
The timeline essentially is:
FRLG approx. = RSE
- 3 years later ->
HGSS/DPP
- some years later ->
BW - 2 years later -> B2W2
- some years later ->
XY
I used the remakes' names for simplicity's sake.
Gen 2 is confirmed to take place 3 years after gen 1.
Because of the reasons I've listed above in response to Salt&PepperDiner, gen 4 and 2 take place essentially at the same time.
Caitlin appears in the Battle Frontier in gen 4, and then in BW as an E4 member having aged a bit, so we know there is some years in-between gen 4 and BW.
B2W2 is confirmed to take place 2 years after BW.
So that leaves XY and RSE.
In gen 5, if you transfer a Pokémon from gen 4, it will only say the region it's from on its profile page. If you transfer a Pokémon from any gen 3 game, however, it will always say "after a long travel through time." This is really our only way to place RSE in the timeline--close to FRLG. Since we know FRLG takes place at least 4, but likely many more, years before BW, it makes sense for the profiles to mention that. The fact that they mention it for RSE Pokémon and
not gen 4 Pokémon is likely not just coincidence.
In XY, however, ANY Pokémon that is transferred says "after a long travel through time and space." This essentially confirms that it takes place some time after B2W2, as there is an NPC who mentions that Team Plasma was on the news, so it can't be before gen 5 (and that Kalos is even farther away from the other regions than usual, most likely).
This being said, I do think the timeline is of great importance, and it will affect the human cameos we receive, and other plot-related things. But game mechanics can easily be retconned (FRLG had Dark and Steel type Pokémon and Abilities, and HGSS had the Physical/Special split) and
will be retconned, so the gameplay is up-to-date. The only things the timeline will affect are those that are story-related.