I have been avoiding this one as I knew it would not be straightforward. Certainly, the 10 current travel regions to correspond with the provinces is too many. The clearest provincial map of Sumatra I have found is here.
I propose a division of this vast island as follows (north to south):
Aceh - leave as is. Although the chances of it ever being a separate nation are greatly diminished these days, this is a special adminstrative territory which very much stands alone as far as the traveler is concerned. Has a quite a lot of good content.
North Sumatra - leave as is. Lots of back-packer driven content and many of Sumatra's leading draws.
Riau Islands stays as is. This is a very different region to any of the others. The highly developed Bintan is basically an extremely naff playground for Singaporeans, while many of the others are so off-the-beaten-path as to get almost no visitors.