I fear the ramifications of this kind of suggestion. You are essentially connecting an alliance all together by 1 specific outpost. Sure, every members can work together to help protect it, but it doesn't mean they will and it doesn't mean it will be used correctly. For instance, now you have a bunch of players trying to micro manage a single place... imagine someone trying to keep to the teams chassis and makes 120 infantry, but another player noticed the attacker sent pure AI and decided it'd be best to make AI tanks. So he decides to rush the units to get the tanks up. Who ends up paying the rush fee? Also, who even ends up paying the initial resources in general? Is there going to be a stockpile? Or is this just a TB with a lot of downsides and 1 perk (all being able to make units here)?
I agree, it would make game play harder... but not in the sense that I think you would want. I feel it would only put more pressure on the one's trying and give others more reason to be lazy (my alliance will put in more effort because if they don't, we all lose out).