NXT blockchain pruning does not even have the concept yet. It won't be easy as some claims to be. If blockchain pruning can be done at will, why do bitcoin not have one already ? So I would say do that first before attempting ping pong messaging.
Who said anything can be 'done at will'? That's specious. And how is Bitcoin, an antiquated 1.0 crypto, suddenly the sine qua non of what can and can't be accomplished?
Bitcoin doesn't have the advantage of Nxt or NEM... smaller cryptos can be nimble. We don't have $5+ billion dollars tied to our blockchains (yet!)
We're testing NXTsubatomic crosschain exchanges *today*. NEM and Nxt don't have committees and foundations breathing down our necks. Or miners.

Good luck with the "subatomic crosschain".