I've played a number of different MMOs including EQ, UO, and WoW. But I've also dabbled quite a bit in City of Heroes and its related games. After a year away I returned to CoH last month to try out its new content and additions.
For those who've never played the game uses a level system but doesn't rely on it for hitting and missing, and recent content – mostly world events – does away with levels for enemies. CoH assumes a 75% hit-chance for even levelled opponents, which can be modified by gear to a maximum of 95%.
World of Warcraft has a similar system. No matter how much weapon skill you have, a certain percentage of your attacks will miss, be parried, be blocked, and be dodged. This overlaps with critical and other attacks and reduces damage, so you need to increase your Hit. You miss higher level creatures slightly more but still hit roughly 70% of the time.
Hitting and Missing
4e essentially assumes a 55% chance of hitting. If you roll 10+ on a d20 you hit the average monster of your level. All the math is based around this and all the level, gear, and stat is designed around keeping this rough ratio of hitting just over half the time. The rate you gain bonuses is all designed around maintaining the established hit : miss ratio.
While playing CoH and fiddling with my enhancements to avoid going too far over the hitcap it occurred to me how dissimilar Zombies! have a similar hit mechanic; roll a die and get above a certain number that is roughly 50%. I wondered why doesn't D&D do the same. Most of the recent changes in the game all revolve around emulating board games, so why not take their hit mechanics?
Roll a d20 and if you get a 10 you hit. Simple as that, remove all the BAB and hit modifiers and math. There's no need to worry about disparity between monsters defences and the like. Fighting a lower level monster? The needed roll just goes down. Fighting a higher level monster? It goes up.
If it Ain't Broke
Why change D&Ds system of modifiers and a die roll against a static number? It's easier to balance for a start. When you know exactly how often players will hit and miss you can use that in calculations for hit points based on the ideal number of rounds for combat.
It's also much easier to explain to new players. None of that "add your stat– not the actual number but the related bonus – to half your level, the bonus of your item, any feats you have, and the proficiency bonus of your weapon. Then add that to a d20." Instead it becomes "roll a d20, if you get a 10 you hit."
It also makes all characters hit evenly. Instead of dumping all your stat points into you hit stat to get an 18 (20 with racial bonuses) you can spread your stats around for a more rounded and interesting character. 4e characters really have 2-3 dump stats with one uber stat; removing the pressure from that one statistic makes for more balanced characters. It also prevents that one min/maxed character from dominating the table. In my game it's a halfling rogue. With 20 dex and an impetus to always have combat advantage he's running around with as much as +3-4 in extra bonuses and making other characters feel weak and inaccurate.
It wouldn't be the first time the to-hit mechanic would be completely revised. Tables were originally used in 1st Edition, and this lead to the slightly revised THAC0 system.
Really, my proposed system is fairly similar to THAC0 with a set to-hit roll modified by the monster's level. Only the set number wouldn't change except when fighting monsters of a higher or lower level.
It wasn't until 3rd Edition when your to-hit number started going up instead of down. A radical change at the time for D&D but fairly similar to what had been done in other RPGs.
Magic and Equipment
The issue with the above is it removes magic from the equation. For all of 4e's work to make PCs less reliant on magic items and make magic less essential to the character, the three big magic items are more essential than ever. You get several powerful magic items before you reach second level. They had to increase the former maximum of magical plusses to +6 to accommodate their new math.
A set to-hit roll means magic becomes less mandatory but possibly more special. Instead of magical plusses magical weapons and items would offer other benefits and side powers. A few could grant plusses to hit and these would be rare and potent. The standard +1 sword is suddenly special. It allows you to face and hit more powerful creatures as if they were your level.
The other issue is how to handle defences. How does armour work in a game where a 10+ always hits? Why wear plate when you're going to be hit as often as the wizard in cloth?
One solution is to make monster attacks work differently, so they need a roll modified by the amour. There's a few ways to balance this but it does make monsters attack significantly different. Another idea would be to ditch the monsters attacking and have the PC roll an avoidance mechanic, so they're actively rolling to dodge and avoid.
The option I like the most is to add damage mitigation to the game. Armour reduces physical damage, a strong will reduces psychic damage, and the like.
My Hero Misses HOW often?
By setting an arbitrary number on the die, hitting can also be increased. 55% is really too low. If you assume half your blows do actual physical damage and the rest are parried, blocked, dodged, or otherwise avoided then half is a decent percentage of hitting. But for 4e, with its assumption that hit points represent dodging, this means you "hit" half the time and they parry and the other 45% of the time you miss so badly the enemy doesn't even have to sidestep the wild swing.
I like 70%. It's a reasonable number and still allows some miss range (in d20 terms you hit on a 7 or higher). And with a powerful +5 sword you still miss on a "1".
This is just some rambling brainstorming, but I think it's a neat idea and probably one way 5e might turn-out if they keep going down the road of taking inspiration and ideas for D&D from board games.