the same O/S on everything
actually I don't think this will be a problem.
How many windows phones and surface pro tablets will still be around by the end of 2015 ?
Now subtract all the older than 2015 devices as they won't be upgradable.
Finally subtract the 99% of users who aren't daft enough to pay for a new O/S.
We are left with 2000 corporate users all based in Redmond or Nokialand and a couple of dozen seriously mad folks.
the only universal apps needed for Windows phone and Surface Pro will be Maps for finding Microsoft offices and a practical guide to muttering on trains whilst dribbling.