Is there any update or extension for AMOS which makes use of the AGA chip or graphic cards?

[alx]

Welcome to A.org :-D

Some people here hate Amos, others love it.  What is true though is that it is completely tied to the Amiga's custom chips, and will certainly not work on OS4, MorphOS or even on Amithlon.  If you're interested, there are a few people developing Amos-inspired BASICs.

As for AGA, there are are a few extensions that might be what you are looking for on Aminet.

[alx]

If anyone can give me a hint concerning tools and languages...

The main language used on OS4 will be C - initially, the only development tool will probably be a modified GCC.

As for BASIC, I believe that Blitz Basic and it's deviants are fairly system-friendly (ie should work on OS4 without major adjustments).  These are languages like Blitz Basic 2, and there's something called "Pure Basic".  But I'd reseach first just to find out.