Some series I want back which have already been mentioned:
- Ecco (2D please!)
- Jet Meto (I was actually thinking of this when I clicked on the topic)
Also, I want to see these come back:

I want Rayman to return to 2D where he rightfully belongs. I don't want to see these spin off party games anymore. Give me something akin to the original Rayman which was awesome. Actually, even Rayman 2 was a great 3D platformer - but still I prefer 2D if they can help it.

Ok, so Zombies Ate My Neighbors can't really be considered a series, but a sequel was released that apparently failed (how come I only found out about this now?). Having this game return would be awesome. Keep it 2D, enable wireless multiplayer, custom level creation, I don't care. Even a basic game with the same concept with minimal features would be great!

Bring back Donkey Kong Country. In other words, a proper 2D DKC game. I just want good platforming to return.
Speaking of platforming, I wish that Disney could go back to releasing awesome platformers based on their movies. Who doesn't remember playing Lion King, Alladin, Beauty & The Best (I can't believe a game this hard was marketed for children!), Mickey Mouse Castle of Illusion, QuackShot, etc. All these are awesome platformers.
Looking back at this entire post, I've basically come to the conclusion that I want to see the return of the 2D platformer.
Also, this too:

It'll never happen though because Konami now owns the rights and they're too ignorant to continue a series that bested DDR and proved that they suck as a company. Long live ITG... at least you continue to live on due to custom song creation.