A portable BASIC in the style of Microsoft 6502 BASIC (especially the Apple ][+ variant), with influence from GW-BASIC and Atari BASIC among others, with enhanced features, written in C for portability to computers large and small.
Exil is a bytecode-language that (when it's finished) can execute GW-Basic programs, after converting them to an own format. Maybe it's possible to add other languages later.