More extended types? (sockets)
SIGINT
Dispatch table for builtins; saving cores
Reorganize tag bits (special/char/builtin sharing etc.)
Flexible heap size

Extensions:
Fixnum: popcount
I/O conditions
Time of day
Sockets: hosts / services file parsing, DNS

Optimization:
Syntax dispatch
Use tries for symbol interning & variable lookup
Some list procedures could build by mutation to avoid non-tail recursion