Seriously, why not make a Borderlands character pack? You could do several, one for each game.
Borderlands pack one would include the four original characters from the first game and another pack for BL2 and one for the Pre-Sequel.
It would be easy. They already have character designs and skins. They already have skills from their skill trees that could be adapted to Battleborn.
This would bring in droves of people who are eager for more Borderlands, increasing sales and filling the PVP arenas with users for the next year or two until BL3 comes out.
The only concern would be how this fits into canon, but this is a video game. We can already play PVE with Melka fighting alongside Melka in Archive or Melka and Deande in Heliophage.
Do it, Gearbox. Save this game.