Why not make it just like with the TC system?
When you have a base, you can decide what hour is going to be your hour for Base Defence (?) and during that time span you better be watching your property. Important here is that the owner can choose the timeframe, since some people play only at specific hours. And also, the strength of the oncoming forces could be dependent on which base you bought. The higher the price, the stronger the attacker. Or it could be dependent on the number of people in the gang. Or a combination of those 2. Of course having mercs for defence would be advised, as well.
If you still think that this would be boring, you could set the timespan to 10-30 minutes, rather than 1 hour. Or even make it happen every 2nd day, or 1 day a week. It doesn't have to be a major feature of the game. Just something to add atmosphere.
I also like like the idea of critters following a player to his base.
You could also implement a handicap period for the first week, where your base would not be attacked at all, and later on a holiday-mode. Let's say One week for every month. After all, its a game ^^ You activate it for 1 to 7 days and again, you're protected against attacks for this amount of time.