There are many different ways to go with this... but this arguably works best if Abathur is to clone Rehgar.

Alternatively I'd advise putting more points into the shield, especially at level 16... for what is better than 4 shields that do damage? Rewind at level 20 might be more valid than upgrading your former ult... it really depends on the game (team comps).

2 healing Rehgar's can save a team and/or chase a team safely.

Abathur Build (warning, wall of text and ramblings):
http://www.heroesfire.com/hots/guide/abathermite-wall-of-text-w-ramblings-clone-locust-strategy-5261