As many have stated in several other forums, many players would like some ability for Marines to aid in the building process. Many of these ideas can also translate to Aliens via the Gorge.
One option is that MACs are the only thing that can place buildings, but Marines have the ability to help build them. This can either be an innate ability all Marines have like in NS1 or you have to purchase a special tool from the Armory.
The other option is that Marines can place some buildings. This would probably require a separate tool and would cost personal resources, or you can only place a certain number of these buildings (see TF2 and Empires).
Similarly, for Gorges they would have an innate ability to help speed up building. They already have a building they can place via the Hydra. Originally suggested by RobB
This provides several things.
An outlet for poor shooters to be helpful by helping to build for their team.
Commanders still have primary control over major buildings.
If you don't want to help build, you can still help defend.
Provides the option of "power building," a very fun mechanic from NS1
EDIT: other suggested thoughts: MACs/Drifters still build the fastest, but Marines/Aliens can help it along [link] Marine tool would be the welder. [link] Players can place smaller, weaker Resource Towers [link] (similar:) Players can only place low-level buildings (my guess would be RTs, Turret factories, etc) [link] Players able to deploy MACs or buy buildings via a MAC [link] Research the ability for Marines to build [link]