Several feature requests involve "allow me to embed <some other markup language> in Remarkup", in some form or other -- to draw sequence diagrams, or UML, or cows. Since these are uncommon and somewhat different from other remarkup rules, we should develop a unified syntax for them and implement them in a pluggable way. One syntax might be:

cowsay (x=y) {{{
block
block
block
}}}

Where cowsay is the block type (e.g., cowsay, plantuml, dot, etc.), (x=y) is an optional list of SimpleOptions which are parsed to the engine (e.g., which cow for cowsay), and then {{{ ... }}} delimit the block itself.