Although the English language follows pretty much the same rules everywhere it is spoken, in order for a language to be an efficient tool for communication it has to be fully standardized (especially in its written form). When differences between the different varieties of a language become too great, the advantages of speaking a common language diminish, as its speakers are driven away from the common standard.
The main varieties of English used throughout the world internationally are British and American English. These varieties would benefit from cooperation between the UK, the US and other English-speaking countries to agree on a common standard of spelling. That shouldn't be overwhelmingly difficult since the differences are small.
And there is a precedent in the world: Portuguese. The Portuguese of Brazil and the Portuguese of Portugal showed many differences of spelling (and from what I understand, to a larger extent than English). So, a little over a decade ago, the different Portuguese-speaking countries reached an agreement which would have most of the language standardized in its written form (with some exceptions).
An international Council of the English Language could be set up to act as a guiding institution.
What could change:
(Variants: selected standard)
-ise (UK) and -ize (US) : -ize most standard spelling since already accepted by Oxford dictionary
-our (UK) and -or (US) (as in colour/color): -or probably better phonetically
-tre (UK) and -ter (US) (as in centre/center): -ter better phonetically and used in BE some time ago
-double L (UK) and lonely L (US) (as in traveller/traveler): in many cases, doubling the consonant is a rule (to put --> puTTing); make this a general rule and keep double L as standard.
-judgement (UK) and judgment (US): reinstate "e" in US form; dosen't really make sense phonetically and historically (recent change).
That's mostly it... What do you think ?