Race disclaimer: I used to race-pick Terran, but now I play random.
Anyway looking at the mechanics available to the three races to defend distant expansions:
Terran has PFs, and can use bunkers walled off by depots to fend off small zergling runbys. But making a PF comes at the cost of mules and not being able to lift off. It also cannot deal with a dedicated attack, however, and Terran must bring their army to face the other army.
In HOTS, Protoss gets the nexus cannon and recall. This is a bit limited in that only 1 MsC is allowed, since even a max energy MsC can recall once and cannon once, but it is better than nothing. Maybe it would be nice to allow 2 MsCs once P has a fleet beacon? An issue for another day perhaps.
The zerg hatchery is however boring as hell. Its a hatchery. You inject it and build spines/spores around it. Apart from the one in your main which you upgrade to a hive, all the others will remain hatcheries until your opponent uses drops, warp-ins or run-bys to kill it.
Shouldn't Zerg get an alternate form or a modifiable form of a hatchery? Something more suited as an out-lying expansion but with some drawbacks. Here's what I came up with in 5 minutes (don't take it too seriously):
Zerg can now choose to morph a hatchery (cost 150/100?) into something that has more health and/or more health regen (Health between lair and hive? 3x normal regen? subject to balance). It cannot be morphed into a lair or hive later on. Added hp and regen seems more zerg-like than a PF which is added hp and armor. This new hatchery however is not injectable (similar to no mules from PF), and spreads 50% reduced creep radius (it retracts creep from normal hatchery form, needs creep tumors/overlords around it otherwise buildings built on creep when it was a hatchery will slowly die). A more drastic option is no creep apart from the 5x5 under it. Also, this new hatch acts as a 1-way nydus worm (exit no entry, ground units only of course) that links up with any existing nydus networks. If you have a nydus network in your main, you can teleport your army to defend your 3rd, but if you send your whole army through it, and there is a drop in the main, you now have to walk back the slow way. If you want to teleport back, you need a proper nydus network exit near the 3rd. This doesn't negate the need for a nydus network, rather it promotes both defensive and offensive nydus play, which I think needs more love. This morph should be available following evo chamber? so that you can morph it at your 3rd or 4th before your nydus network is complete (i.e. the nydus network should not be a pre-requisite to gain the benefit of added hp and regen).
Why? hatcheries surrounded by spines and spores in the late-game are boring, and some zerg compositions have trouble defending multiple bases. If you have a nydus network and if you still have a small amount of ground units e.g. roach/hydra and a few infestors that you are willing to split off from your slower broodlord/swarmhosts, you have more options to defend bases instead of relying on fast units like muta-ling. More options are better. Things that split up armies are also better. It doesn't make Zerg defense hole-proof because if they mindlessly offload their whole ground force in one place, they can no longer defend another area quickly (due to 1-way nature). So in order to use it properly, they need to offload only as much unit they think they will need to defend a base from e.g. a drop.
TL;DR:
I propose a new hatchery morph (conceptually it is a Zerg PF that uses units to defend instead of a cannon):
Pros:
More health and health regen
Acts as a 1-way nydus exit for ground units
Does not negate nydus play, instead promotes it since you need a nydus network anyway
Cons:
Not injectable
Reduced/no creep radius. Need other forms of creep in order to build spines/spores.
Terminal evolution (Cannot morph into lair or hive).
Requires a nydus network to use nydus exit functionality
Requires evolution chamber for morph
Please post any constructive criticism you have. I didn't think it through too much. Is it absolutely OP (concept wise)? Or do you think it is valid, and can be balanced by tweaking the numbers (cost, health, regen, build time, max unit unload speed)? Or is it too complex?