The victory conditions lead to games ending with a whimper rather than a bang. I'm not exactly sure what the answer is, improving the AI should help but you hit a tipping point as soon as you control more than 60-70% of the map at which point victory becomes an inevitability. Maybe it would help if there was a maximum amount of support you could have on any one tile at a time, so that one way or another you would become vulnerable to attacks on your capital. That limit would have to account for entrenchment bonuses of course but it would lead to games ending with a decisive attack on the capital more often.