I think I suggested this once before, but I wasn't able to find it in a search...
This change would provide the motivation for defending a base. Positive and negative reinforcement.
You can build structures, forts, etc. anywhere you want. You can escape hordes with a vehicle. Once you install a Colony block however (similar to a LCB), you've designated this spot a colony.
What happens...
1) Your colony block attracts NPCs.
2) The cap on colonists would be based either on your level, or a perk, but the total would increase very slowly to reach that cap. Quests might include rescuing people that would then become colonists.
3) Depending on TFP's abilities and desires, NPCs would either not be able to defend themselves, or perhaps have limited capabilities to run or melee.
4) The NPCs would provide passive abilities and benefits which might include auto-farming, running forges, or giving certain bonuses or perks to the player just by being in his colony.
5) Special NPCs would exist which might provide extended or unique abilities. For example, if you have a colonist with the "Well Digger" ability, he or she will provide you with a well kit which will allow you to make an unlimited water source (not necessarily fresh) within your base. A good cook might give you a stamina bonus. A doctor might quickly patch up your wounds or remove most debuffs. You might even be fortunate enough to attract your own Trader!
6) On Bloodmoon nights the zombies will beeline for your COLONY, not you. They'll attack you, your base, and your colonists if you let them.
There's a TON of incentive to defend your base. You'd even need/want to design it differently, and much more seriously if colonists' lives were at stake. If TFP could implement a combat capable NPC colonist, BOY would that be fun. Nothing would be stopping you from abandoning the people you swore to protect to save your own skin, but your people would be very dead and much of your colony structure decimated. All your bonuses gone, and the unique ones probably gone for good in that game. This would definitely add some RPG zing to the game for those who like that, and a set of special perks to earn and protect for more spartan gamers. It's sort of how FO4 was supposed to do settlements (but with some actual lethal danger to the NPCs).
I'd suggest that the zombies would attack and eventually destroy your Colony block and then just wander the site until morning. Until then, your chests full of goodies might be lost, but I'm not suggesting the zombies would actually seek them out either.
-Morloc