Don't think Black Templars are the SMs in the Dark Millennium MMO, unfortunately..
Obviously they and the Dark Angels will get updated eventually, but there's not really too much that can be done to differentiate them from vanilla marines past what they've had, it's more about balancing I'd say.
I mean, they already have some unique models and characters, but what can be done further without something LOOK-AT-ME BIG like the Stormraven for the Blood Angels?
I'd imagine Dark Angels getting some form of very-plasmaed Land Raider or Predator..
As far as Necrons go, I'm expecting them to crop up before Tau, we know there are plenty of rumours flying around, most of which are fairly bang on the money.
Necrons are easier to get out since there is less established background in their sole codex, meaning there are less toes you might tread on when adding/retconning.
Plus, whilst their look is fairly standardised, there is still a lot of room for change. Since Tomb Spiders are coming in plastic, there is also the option to change the look of the Destroyers too.
Think of how many different designs there have been for Juggernauts of Khorne? All of a similar style, but slowly and subtly refined.
This is how the Necrons will hopefully be handled.
Tau obviously have a very set style, and a fair amount of units, but where do you go without adding more non-Tau aliens?
The main rumour to come out for Tau is obviously that Crisis/Broadsides will be changed due to their snappable ankles, but past that?
Who knows..