The problem is Wasteland, straight-up. If they waste your Shelldock Isle, you have to try for a funny storm plan. It rarely works out.
You could go with a mana disruption package with Stifle.
On this list I'd see...
-4 Cabal, +3-4 Null Rod
-3 Shelldock, +3 Wasteland
-4 Duress, +4 Spell Pierce
-1 Scroll/Tinker/Sphinx, +3 Stifle
-4 Vault, +4 Confidant/See Beyond
-1 Chain, +1 Massacre/Infest
I'm not sure if I'd run an alt win, since Doomsday by natures is very committal.
Can we please try to be constructive here, this is in the development forum for a reason. That doesn't mean it should be trashed and I should build fish

.
This is one of the most fun fast and easy to play lists I'm come across since flash hulk. The only thing I could possibly think of that would make it better would be 4 gush.
In a different direction; does any one have any thoughts on adding an oath plan? Theres only two creatures and they're both possible oath targets, shelldock isle also has nice synergy with oathing.
? I
was trying to be helpful.
If you don't like the suggestion, feel free to not use it. I'm just brainstorming ideas here.
And my critiques aren't about "making it fish," but rather focusing on:
1) you don't need artifact mana for your combo so Null Rod puts you in a good position against those that do.
2) Lim Dul's Vault is generically a card that I don't like.
3) Reactive disruption (Spell Pierce) is better than non-reactive in this case.
4) I don't like Cabal with so few Moxen.
5) You never want to draw Shelldock more than 1 is a bad idea.
6) You need a maindeck board wipe card over a singleton bounce to keep a decent amount of life so you won't die post-Doomsday.