I've completed all the quest lines except the Deadric ones.
It seems like I've been everywhere and regardless of how much I walk around, I can't find anything juicy to trigger a new quest (of any significance)
There's nothing significant in my quest list to pursue.

But I want to finish the deadra quests and bag the platinum.

Any ideas what I should do?

I know you'll probably ask "well what deadra quests have you done so far?"...

And tbh I couldn't tell you which ones I've done

Here are some hints on where to go:

There is a Viligant of Stendarr in Markarth that needs help with a haunted house.

There is a crazy old man in Solitude who is chanting about a hipbone.

There is a Orc Stronghold in the Rift that is in need of some help.

One of the Jarl's children in Whiterun is acting strange.

Dawnstar is under a terrible curse of endless nightmares.

Something is eating the corpses in the hall of the dead in Markarth.

There was a killing in Falkreath.

An Imperial in Dawnstar has an open tour of the Mythic Dawn.

I found a bunch of quests in Settlements and Villages. Some even led me to places I have never been before.

However, the game has tons of Un-marked quests and Collector Quests. These will not show up in your Journal, and sometimes the reward is just doing the quest.

Here are some Journal Supported Quests:

http://elderscrolls.wikia.com/wiki/Side_Quests_%28Skyrim%29

Here are some Misc Quests that you are given, but no Journal (usually)

http://www.uesp.net/wiki/Skyrim:Miscellaneous_Quests

I don't know if you're only looking for quests, but I've triggered events just by wandering around.
For instance, there's no quest connected with Angi's Camp but it's definitely an event. Same with Kagrenzel.

Lots of caves and places of interest have a cause-and-effect string that pull you in for one reason or another, maybe starting with just your own curiosity.
On my 4th character, at level 70 (seemingly for months at this level!) that's almost the best part of the game world, and it's what keeps me wandering around.

Markarth and Dawnstar both have 2?

Markarth

Dawnstar