Concept: It's a Trap!
A fairly bulky Pokemon that is meant to utilize the underused trapping moves available to us. After it has trapped its prey, it begins to kill by indirect means, hopefully Perish Song.
Concept: Missey
I read a post once that described what I believe to be the accurate reason that Blissey is not an overcentralizing force. You can use a combination of Zapdos/Heatran/Celebi to cover most every Special threat in the current metagame... and it will be a fairly THREATENING combination of Pokemon. Or you can use Blissey to sponge up attacks, but generally do nothing BUT sponge, as it is just plain not a real THREAT beyond status inducing. I think a physicaly defensive counterpart to Blissey would be a good goal, as you could use a combination of Gliscor/Hippowdon/Skarmory and cover most all physical threats, and be a fairly threatening combo yourself, or you could use the non-threatening sponge that would be "Missey" (Tentative name obviously)
Concept: Me First
I have always loved the idea of using Me First, but it's not on any pokemon nearly fast enough to take advantage of it. This pokemon should be defensively frail, but have adacquate attacking stats in order to take advantage of moves on both sides of the spectrum... The main focus should be on speed, though. It should be able to outspeed most, if not all of the metagame in order to use the move Me First viably. It should be noted that its movepool should be fairly limited, as the primary use of the stats should be abusing its opponents moves, not its own.
Concept: Wait, where's my wallet?
A pokemon meant to "steal" the opponent's strategies... Should be able to utilize the moves Thief, Snatch, Mimic, and with ease. It should have access to most of the Stat Up moves, but have fairly mediocre stats as bases for boosting. I know this isn't the most in depth analysis of the concept, but I'm open to suggestions on how this is supposed to function/work. I just really like the idea of a pokemon who steals.
Concept: New Weather Effect
This particular pokemon would be made with the idea that it would benefit from a newly made Poison type weather move... Something along the lines of hurting pokemon for a certain amount of damage each turn, while healing Poison types for a certain amount each turn. Obviously the community would have to come up with something not broken together.
Concept: I know what you're doing, and I'm not going to allow it!
A pokemon specifically designed to figure out the opponent's moveset... With all of the problems arising from multiple movesets that certain pokemon can use that counter its own counters, this pokemon should be able learn its opponents moveset quickly and efficiently.... and hopefully, it should be able to do something about dismantling the opponent's moveset in some way.
So there you have some of my ideas for good concepts... hopefully you all like them too!