Something that might be really sweet would be if artillery was implemented as per the venerable Fantasy General (and Panzer General) system. There, having a piece of artillery in range of a hex (province) where you were attacking meant that the artillery would fire in support.
XX | xx | oo
If XX is the hex/province with the artillery, it would fire when the units in oo were attacked by friendly troops. Similarly it would fire in support of the troops in xx if they were attacked. (Not positive about that Last - it's been a while (years) since I used the mechanics.)
So, artillery might be treated as a size 6 unit (thus eating lots of food) which acted as outside archers in a battle. Maybe with an area effect 1 attack, and all the headache that involves when the artillery kills your own units.
Edit: Artillery like this (firing outside the province) might also go a long way towards balancing raiders and flyers, if it fired before movement, thus could inflict some damage on flyers and even stealth troops before the movement phase.
To balance that, maybe you could only build artillery units in provinces with fortifications. That _might_ encourage more castling, but with them having a range on them, you wouldn't need a castle in every hex. And, especially if they were AOE 1 weapons, only have them fire every couple (2-5) turns, possibly even only once a turn. So, you could fire at the provinces in range of you, and/or they'd fire as combat support in provinces in range.