Ok may I suggest a few things with this....
First if you're going to rewrite the whole script and add some of your own functions how about making a script packs based on Type of the RPG a person wants to make, for example - "Golden Sun Pack, Pokemon Pack, Standard RMXP Pack, Standard RMVX Pack, etc.", which contains customized and optimized scripts just for that style of the game, instead of making All-in-one script pack.
Second, could you use Thread.new in when rewriting scripts, i think it should pretty much optimize threaded function allowing you to do more stuff at once without putting a burden to processor.
#Thread is a built in class in ruby
c = Thread.new{block of commands}
# Refer to ruby API to see functions of Thread class
Third, if you need my help in something I would gladly help, I'm not much, but I can say I made a pretty decent Tilemap Class (RMXP one) (which is not yet posted), without priority 1 bug and works pretty good with RMVX (except for really large maps, it slows down a bit when loading them, but only during that).
Fourth, how about making some (if not all) the scripts be Multi-Platform (or should I say Multi-Engine)? Like being able to be used in both RMXP and RMVX? I know you're making a complete rewrite so why should you, reason is since RMVX is has more optimized Graphics class which uses DoubleBuffering in full screen mode + various fixes. The way you could obtain this is...
Since RMVX uses $TEST global variable and RMXP uses $DEBUG global variable it is possible to achieve this by using this method:
# If RMXP execute this code
function_to_execute if defined? $DEBUG
# If RMVX execute this code instead
function_to_execure_vx if defined? $TEST
Well just some of the ideas I had, hope you'll find use for them.