Hi Rudolf.
I think it is not a good idea to design your own language. jPDL should be capable to catch all your requirements. And jBPM 3 has even a more language possibilities than jPDL 4 by now. I haven't seen much requirements it could not implement.
For BPMN 2.0: That's the way to go for the future. But at the moment the engines are not yet far enough in my eyes. Definitely interessting, and if you have some time bevore you have to go live, than I would consider this option.
And even XPDL would be a better choice than an own language.
If you are Geman speaking you can find some more oppinions from me on that here: http://www.bpm-guide.de/2009/08/02/bauen-wir-uns-eine-bpmn-20-engine/
Cheerio
Bernd