Having full access to an entire pool of options, not all of which suck, is strong.

Basically your powers become the best power of 3 pools of powers instead of from 1.

The power-swap feats, which require your multiclass, cost a feat, and let you swap 1, are reasonably strong feats. Even if you only ever swap 1 power in each category, this is as strong as 3 decent feats.

Throw in attribute-swapping and it could get ridiculous.
About your only defence from this being top-tier is the lack of charop work on it; your players will have to do their own charop. They can still just look up the best powers from wardens and druids and see if they boost their build.

...

You could split it into a pair of Theme's and a racial ability. That might help.

A "Warden" Theme tied to the race, and a "Druid" Theme tied to the race. Plus the racial attack power might be "pick one from Warden or Druid" with stat-swap.

That would mean you couldn't for example, abandon your original class's powers and take all Warden or Druid powers as an Avenger.