I got to pondering what Japanese might end up like if it were to be
exported around the world and picked up by lots and lots of
second-language speakers. I am somewhat inspired in this by the
proliferation of American fans of anime; that phenomenon, combined
with the large number of English loanwords into Japanese, would seem
to make an excellent environment for a sort of English-Japanese
creole, if, say, a whole bunch of American otaku were exported to Mars
or something.

I don't have much so far, but the most obvious change I would expect
is a loss of vowels from original Japanese words, resulting in new
consonant clusters. Specifically, the loss of devoiced vowels, which
English speakers often just don't hear.

Some additional tweaking to the rules may be necessary to avoid
creating clusters that the Anglophone bad-Japanese learners would
themselves find difficult to pronounce, but the basics would be
something like this:

1. No two vowels in consecutive syllables can both be dropped.
2. /i/ & /u/ are candidates for dropping following a voiceless
consonant at the end of a word, following a voiceless consonant
following a downstep in pitch, or between two voiceless consonants..
3. /o/ is a candidate for dropping between voiceless consonants where
it precedes another syllable containing /o/.
4. When two or more consecutive vowels are both candidates, which
vowels are actually dropped is determined by dropping the right-most
candidate vowel, then the penultimate candidate vowel, and so forth.
5. Dropping of final vowels results in compensatory gemination of the
preceding consonant, and aspiration on stops.

After this phonological change, there is the possibility of continuing
to write original Japanese words according to their historical
pronunciation, but given that consonant clusters are now allowed,
there is the possibility of loanwords which lack vowels in places that
cannot be predicted based on those rules, and so some means would have
to be devised to write individual consonants.