Right. I #IV in C major, for example, is C E G to F# A# C#.
But you can take the I #IV to say, A major, and get A C# E to D# Fx A# (Eb G Bb). It'd be the same thing concept wise. The numbers keep it true to itself no matter what key you play in.
As for you own vocabulary, think of what we have now. We have cadences like V - I = authentic cadence, IV - I = Plagal, etc.
Suppose you used ii iii in your music a lot and used it in cadential places (at the end of your phrases). This doesn't have a name as a cadence, so you can name it your own thing and use it over and over (if you use it well) and it'll become a unique sound in your vocabulary.
You can do this with chord progressions (||:ii iv Ger+6 V:||, for example) and name them your own things. Then you can use them as full ideas and mix and match concepts rather than individual chords and notes.
In another perspective, think of notes. Now if you take C E G, they're just 3 letters. But together, they equal a chord - C Major. Same thing with grouped chords.
A major and D minor are just chords. But if you go from one to the other, you can say this I - iv relationship can be named something; maybe the Blastingas10 Duo. Then you can use this concept as a whole somewhere else. So your chorus could be a 'blastingas10 duo'.
This is a rather simple concept; but it's like a lego set. You have all the individual pieces (blocks, people parts, etc [[notes, chords, melodic figures, rhythmic figures]]) but if you put them together in unique ways, pretty soon you have the Vatican built. Then the Vatican model is a concept in its own, that you can mix with other large concepts, and pretty soon you have all of Rome. Then you have all of Italy if you combine Rome with something else. But it all gets broken down into smaller and smaller pieces if you want and you're left with a single block or note as the basis of the huge work.
In having your own vocab, you could create works based on maybe 3 main concepts:
'Blastingas10 duo', a 'musicbanter flourish', and then a sequencing 'venjacques formation'. All of this is of course gibberish (for now).
And this would be the same as, perhaps, saying ALL the chords and figures throughout the whole thing. 3 pieces instead of thousands.
Did that make any sense? o.O