Personally, I'd love to see Lv. X's of the Johto starters (which I'd be willing to bet are already on the horizon), Shiftry, Espeon, Umbreon, Tyranitar and the Legendary Birds which I'd say are overdue considering their role in Platinum (the reprints in PotF are still a welcome nod though).
A good Lucario with a Lv.X and Froslass that causes disruption and has synergy with each other...
Of all the Pokemon out yet, It would be interesting to see:
The Johto starters Lv.X's
Bellawesome Lv.X (One that can have a very effective deck build around it that doesn't need precious bech space...)
Furret T Lv.X (T for Trainer...and it's SP...)
Pikachu Lv.X (wOOt, premature stage Lv.X FTW!!1!)
Buizel Lv.X
Scizor Lv.X
A Platoon card... Lucario and Froslass PT Lv.X!!!
Weavile Lv.X
Espeon Lv.X
Umbreon Lv.X
Any Shiny Lv.X!!!
Ninetales Lv.X
Surfing Pikachu Lv.X
Flying Pikachu Lv.X
Pichu Bros. Lv.X
Wailord Lv.X (MONSTER HP!!!)
Mightyena Lv.X
Wigglytuff Lv.X
Mew Lv.X (One that can make Mew useable...)
Castform Lv.X (For each form...)
Deoxys Lv.X
Jirachi Lv.X
Celebi Lv.X
Ho-Oh Lv.X
Lugia Lv.X
Skull Fossil Lv.X (XD)
Swellow Lv.X
Sneasel Lv.X (I'm imagining the artwork right now...)
An SP Froslass Lv.X that moves all of the opponent's Energy to another Pokemon and snipes for 60, no drawback, and a Power that can cause Sleep by flipping a coin and causes the opponent's attack to fail!
An SP Lucario Lv.X that combos with the Froslass...
Heh, I was thinking about a possible Wailord Lv. X and how much HP it would have.
My favorite Sinnoh Pokémon is Drifblim, so I think we could use a Drifblim Lv. X. Many of the Drifblims I'm seeing so far have potential...but they feel incomplete.
I'd love to see a Plusle Lv. X and Minun Lv. X too.
Yeah, I don't care much for the big tough Pokémon. Give me Lv. X cards for cute little Pokémon, and I'll want to get them. I'm glad they're making a Flygon Lv. X too. Flygon is big, but it's still cute.