Have a Premium option in the character creation screen for a Character Generator and a Builders Sandbox (Free to VIPs.) You could build a Heroic character anywhere from Level 1 to 20 in relation to the account content you've unlocked i.e; races and classes, build points, and then access an instance-like sandbox where you could test your build against conditions that they would face in the game. There would be no XP or loot rewarded and nothing from the sandbox could be taken into the main world, including your build. If you like your creation, you have to roll a new character as normal (using vet status or from level 1) and advance your build the old fashion way.
It would be for character planning and build testing only.
There could be a few store options, for example:
* Heroic Character Generator and Builders Sandbox I (levels 1-20, free races and class+what you've unlocked, basic sandbox test area like Devils assault+some specialized simulations like traps, locks, and other class tuning crucibles.)
* Epic and Iconic Character Generator and Builders Sandbox II (Same thing but for level (15)20 to cap using Iconics and EDs if you unlocked them, Sandbox could have a little more variety or actually let you select a privatized instance from a short list of the content you own.)
* Veteran Builders Sandbox (Depending on how the sandboxes are distributed with the Builders I and II kits, the Vets Sandbox could have anything from allowing you to select more test content options, to attached turbine made (not player made) plugins that can provide extended data on combat, effects, DPS/Heal/DC meters and things like that, to maybe even a PvP duelers corner where players can make builds and then duke it out with other players in a off-world setting.)
I'm not sure how to handle gear... maybe its as easy as typing the name of an item in your chat input and it appears in your characters bags to use until you log out, or maybe you get it from the sand box content and have to store it (which could open another tier of bank slots and share storage to purchase) or a combination?
I know I would buy Heroic CG-1 and Vets Sandbox right now - today - and I would pay basic expac prices for them ($30 to $40 a piece - $50 bundled.) And I'm sure I would buy the Epic/Iconic kit later - when I start running that stuff in game. Plus several drops on storage slots....