I can't tell if you are speaking of a world in which reduce [x x] can never return [10 20] or not.
If you do believe that should be possible, then there are a lot of associated issues. If you don't believe that should be possible then there are different issues of ways people have expected to use Rebol that won't work--and perhaps they shouldn't work, but these boundaries have not been articulated.
There are still basic challenges with the paradigm to feel through. I just added CONTINUE and BREAK to be aware of what loop they are breaking (e.g. specialized to the frame of the loop)
Definitional Break and Continue... the Time is Now
Binding has to answer the question of how the words connect to the right loop, in deeply nested structures, with nested loops. Historical Rebol just walked code and mutated the bindings (e.g. in for-each x data block any X words in that block would be destructively rebound to an X with the FOR-EACH. Even if you PROTECTed the block, it was still changed.
I have some avenues of solution to these problems but none of them crack the case of people restructuring code arbitrarily. Chains of inheritance can form loops when you take something out and put it back in and evaluate again.
Anyway, I'm interested in seeing how you might propose solving some of the issues. You have relevant experience and are picking up on it quickly. I do think you will find puzzles in the medium on your own--as I did--but now there's more groundwork to build solutions on.