Space Marines are space marines. They have a strong army and are a tough faction. At the moment they have all the units and abilities to back this up ie drop pods, flags, asm, stun, nades, flames, etc. Fine, don't go head to head with them as elder or you will most likely die without some serious acrobatics.
Elder on the other hand are a moving slightful race. We don't want to be just another SM race with different colours such as "slow for reapers" we want to be unique and be Elder. Relocation is a good start for their power but if you move your base especially in the early tiers and the SM army move in you will more than likely lose it.
I have an idea with the webgates. Give them a shield that will build over time (perhaps paid for). After it has completed it gives extra abilities such as:
-extra shield to the gate
-a larger circumference for fleet of foot
-gives reapers a shorter setup/pulldown time (and even perhaps gives slow-T2)
-a recall ability like on the HQ (perhaps a T2 ability)
...that sort of thing
It maybe difficult due to all the rebalancing taking place but with all the balances elders individuality is slowly disappearing and its just another army fighting another.