Had a thought, marines could start in an off-map teleportarium room aboard a strike cruiser (and as ultramarine strike cruisers are pretty standard, only 1 room would be needed to be built and could be dropped into any map default to ease mapping), instant respawn after death but players must wait for the teleporter there to be recharged,in addition the teleporter there would default to the most forward captured point so as to keep reinforcements coming to the front. As for nids... well they burrow and tunnel and such to avoid surface fire in transit  (the trygon bio-titan fulfills this role as a transport and tunneling machine), perhaps have them start ina series of caverns, randomly, underground and claw their way up through breakable rocks... (these too could be standardized but why bother when a cavern and tunnel and some rocks is so simple) so as the map progresses the tyranid respawns become slightly faster as their forces are moving ever-forwards, thusly making marines have to try to hurry up and press the advantage their deep-strike teams have in rapid deployment to seize objectives quickly and get the PDF involved to help defend them... before the tyranid main lines arrive!