Well, this is currently highly speculative. What I proposed to Andreas was essentially a model based on Eiffel's SCOOP (with some additional influence from Erlang). Whether it's a practical design remains to be seen.

Note that shared, lockable heaps need not be heavyweight structures. It is entirely possible to imagine a shared hash table with one heap per bucket and fine-grained locking, for example. Collections for such small heaps can be fast because the number of roots is limited, and (depending on what invariants you guarantee), you can even forgo stack scanning for most collections or limit the number of stack frames that need to be traversed.