hong said:
A blend of a heavily armoured tank and a lightly armoured, fragile striker.
You want someone to switch between the two styles? Like say "Today, I'll leave my plate mail at home and wear leather armor, since we need to sneak a lot..."
Or do you want a Heavily Armoured Tank Striker?
The second sounds... broken.
The first sounds
impossiblehard, but doeable. (It wouldn't really be "effective" in 3E, either, but maybe you had a certain illusion it was this case.)
But maybe there is an approach. Set your "favored" style first. This decides whether you pick Fighter or Rogue, or possibly Ranger (Two-Weapon Fighting is what I'd suggest.)
Then take weapon and armor profiency feats as needed (starting with Fighter definitely seems easier), and then pick the multiclass feats for the other class you wanted.
In "Defender" mode, you will focus in using Fighter encounter and daily powers first, in "Striker" mode, you will focus on using Ranger/Rogue encounter and daily powers first. Mix At-Will powers as you see fit.
I think from the know abilities of Ranger and Rogue, you should definitely look into those powers that let you shift with your attacks or shift larger distances. Movement is important for the Striking stuff, and your Fighter powers will probably take care of the damage you inflict. (Even if you don't achieve the heights of a pure Striker).