Gandalf - Yeah, it's rare for us to agree on much. The AI is one of the few points we can agree on. I've never felt the dominions AI was quite as bad as a lot of players make it out to be. It can challenge you for a while, especially if you don't exploit its weaknesses too much.
Other things that make an AI nation better - good PD. I played an SP game against the Dwarfs with Brettonia once... their tough PD made it *very* hard to raid safely thanks to all the crossbows, and if they have slayers in their armies they can kill some powerful things with just one lucky hit. So tough as nails PD should be on the list... as well as high-attack units that do plenty of damage.
I would go even a step further with the national summons, and do the same thing to their recruitable thugs/SCs. Then take away their magic so they can't cast spells uselessly (or give them plenty of reinvigoration so they can't knock themsleves out). Then reduce their price so the AI is more likely to hire them. Getting the AI to cast national summons could be hard... keeping them cheap is probably the best way to do it, but there's no way to force them to do what you want there as far as I can tell.
Recruitable high-research mages are a must, as Gandalf suggested. The AI doesn't do TOO bad at researching early on, but it tends to get pretty far behind by mid-game. The removal of chaff units, also a must.
Oh, and the vineshield effect actually is moddable
