Originally posted by Tarn:
I was thinking about this recently... Not because I wanted to bring magic into Traveller but because I wanted to use lifeblood/stamina in d&d3e.
1) Count AC as AR of equal level... Full plate has AC 8--that would suck 8d6 off of the typical fireball. This would take care of most of your problem.
I too have been considering this - I was looking at halving the AC bonus to determine AR simply because most melee weapons roll only 1 to 2 dice for damage, and the only way to up the dice is through a critical hit. In T20, dice can get added to damage through burst fire or from scaling between personal, vehicular, and starship weapon scales - these things simply are not available as options in a Fantasy D20 game.
If you halve the AC bonus, then plate mail has an AR=4, which means the average dagger is going to bounce off unless it gets a critical (which makes sense). If you go the AC=AR, then plate has an AR=8, meaning that most 1-handed melee weapons bounce off (since most of themm use a 1d8 or less). Obviously strength mods may just breach the threshold by a point or two, but this will be rare.
As a neat twist, I also planned on using the magical bonuses on a magical weapon as armor piercing (thus that trusty ol' +1 sword will actually reduce the AR of your target by one, and a +4 weapon is truly awesome). This fit some of the more dramatic stories where magical blades seem to cut through armor like soft butter.
2) Secondly limit the damage of spells. I was using con only for hit points for a while (Lifeblood without stamina). I found that limiting the damage of spells to the that of the caster level at which they were introduced also balanced things well.
I have not figured out how magic would work with the AR and lifepoint yet. Most of D&D's ranged damaging spells are based on range touch attacks (thus ignoring armor). Applying AR normally against the damage dice may fit (as spell damage tends to roll multiple dice), but only if spell damage is going to go against Lifeblood (thus unarmored targets of a spell ar TOAST).
This is going to make magic deadly, but then again, using Lifeblood and AR in melee should make the game deadly too. Whether it balances or not is really up to play testing to decide.
