It'd help if they had a bit of consistency
Well, you're the one designing the system. You can make it as consistent or inconsistent as you want.
I also just really like the impact of a bell curve on mechanics.
Again, I refer you to Arms Law. The core of the system was about 60 pages of text, one for each type of weapon in the game. Each page consisted of a table that gave results for each possible attack roll against each type of armor. The rationale being that not all weapons would be equally effective against all types of armor, and not all damage curves are equal. For example, someone wearing soft leather armor is probably more maneouverable, and would have an easier time dodging an attack from say...a mace, than someone wearing heavy plate. But the plate would offer more protection. So, comparing the two armors on the mace table, an attacker would need a much lower roll required to hit the defender wearing plate. Maybe he'd only need a 37 to hit full plate, whereas he might need to roll 62 to hit the guy in soft leather armor. However,
if he hits, the plate will absorb a lot of the impact, so someone who rolls that 37 and hits the guy in plate might only do 1 point of damage. If he rolls a 62, he might only do 3 points of damage. If he rolls those same rolls against the leather, the 37 would be a miss, but the 62, which is the bare minimum he needs to hit leather at all might do 8 damage.
Since the minimum to score a hit against any armor type with a mace would be 37, and to accomodate bonuses to rolls, the chart for mace would range from 37 to 150.
Crits were attached to the damage values, and were similarly scaled to weapon/armor combination, and referred you to another chart based on attack type: crush/pierce/slash, and each crit was ranked in severity from A to E. So, if you managed a 129 with the mace against the plate opponent, the value listed might be 6BC, which would mean 6 points of damage, plus a roll on the B crush table. Whereas 129 against the leather wearer might get you 14DC, 14 points of damage plus a roll on the D crush table. Both direct damage and crits can be scaled easily, and the value of armor can go way beyond simply "hit or miss." Of course, a crit chart like that is optional if you want to keep all attack damage in a single roll. But crits in arms law were able to apply additional non-damage effects. Stuns, disarmament, temporarily attack penalties, knockdowns, bleeds, etc.
Also, fumble values varied by weapon. It's a little bit silly to suggest that a trainer fighter would fumble on 5% of all his attacks (rolling a 1 on a d20) and it's probably more difficult to fumble with a dagger than it is with a bola, for example. So...maybe only a 1 would fumble with the dagger, while anything from 1-8 could be a fumble with the bola.
My main problem, conceptually, with percentage based systems is that I'm aiming for a technically open-ended system (something that doesn't work so well with percentages) that allows you to totally ignore the random number generator for tasks that are, to you, trivial, but might not be for another character. So a sufficiently strong person shouldn't even have to roll to break down an iron door, for instance. You can't model that situation in a percentage-based system effectively.
Sure you can. Very easily. Not all tasks are equal. So you, as the gamemaster apply modifiers based on circumstances. For example...let's say the stength scale is 1 to 100, and attribute tests are done on a straight percentage basis. That doesn't mean that anyone with a 100 strength automatically succeeds at every strength test. You could, for example, decide that a particular feat is difficult enough to warrant a 30 point penalty. So...let's say there are two players in the party, one with a strength of 45 and the other with a strength of 100. The target is strength minus the penalty of 30. The player with 45 strength needs to roll 15 or lower to suceed. The player with 100 strength needs to roll 70 or lower to suceed.
Or, let's say your strength scale is a more traditional 3-18 system. Then you simply assign percentage bonuses and list them on the character sheet to the right of strength. Say...every point above 14 gives a 5% bonus, and every point below 10 gives a -5%. So, against that same iron door, let's say you have one player with 18 strength, one with 10 and one with 7. You, as the gamemaster assign a success target. Say...60%. A player needs to roll 60% or better to succeed, but they add their strength bonus to their roll. So the player with 7 strength rolls a 35, subtracts his -15 for a net 20. He fails. The player with 10 strength has no mods, he rolls 59, he also fails. Player with 18 strength has +20, he rolls 42, adds his +20 for a net of 62. He succeeds.
Related, with arms law, I always gave out the charts to one of the players and designated the GM's assistant for the night. We'd know which weapons were in use, so we'd pull out the corresponding pages from the book so he only had 3 or 4 charts to look at, plus whatever weapons I assigned to monsters. I would tell him privately the defense modifiers of monsters, every player would give him their character's defense modifiers, and he would handle the math. Each player (and I, as GM, when rolling for monsters) would make our own attack rolls and apply our own modifiers, tell him the result. He would subtract defense bonuses and give the result from the chart.
Example:
Player is attacking orc #5 with a broadsword. He rolls a 87, and his bonus is 15, so he says "102." GM's Assistant knows that player is using a broadsword and that all the orcs are wearing chainmail so he already has the broadsword chart in hand and is looking at the chainmail column. The orcs has a defense modifier of -5, so he looks to 97 on the chart and reports the result: "5 A slash." GM subtracts five damage from the orc while the player rolls his critical.
Remember, each player is controlling one character, but you as the GM might be controlling a dozen monsters at once. You don't need to be bogged down by doing all the combat math too. Players have a lot of downtime between turns. Let one of them do it.
or target percentages higher than 100 (how do you even roll that?)
I think this question was answered by the above, but it's handled by modifiers. For example, if something is terribly difficult, and a player needs to score a target of 110, that means he rolls, adds his relevant bonsues, and the net result must be at least 110 to succeed. If it's a strength test, and using the 3-18 attribute system as an example, then a player with 18 strength gets +20% to strength tests. So if he rolls 70, his net is 90 and he fails. If he rolls a 90, his net is 110 and he succeeds. In this case, to reach a 110 target, a player would need at least 16 strength to have any chance of success, because 16 strength gives a +10% modifier. So, a natural 100 plus the 10 strength bonus would give the 110 needed to succeed.
Also and/or alternately, you could incorporate a "natural crit" system if you want. For example, on all natural rolls of 96 or better, the player can roll percentile again and add the result. The charts in Arm Law by default went up to 150, but any natural, unmodified roll of 96 or better (equivalent of rolling a natural 20 on d20) you roll again and add. So...for example, let's say somebody has an awful strength and is using a weapon they're not proficient in, so they have an attack modifier of -35. But they roll a 97. So they roll again and get 41. So their attack roll is 97+41-35 = 103, which happens to be above 100. Again, players do their own math.
It'd help if they had a bit of consistency, which could be achieved by simply saying "rolling lower is always better", though.
That's simply a design issue. I gave examples both ways, but this is your system. You can easily design things so that whichever one you want is always preferable. Just because attacks use d100 doesn't mean attribute tests need to. You could mix and match if you want. Or you could keep everything on d100.
The point of rules is not to be difficult for the sake of difficulty. Make something that suits your needs. Arms Law was a very
fun system, but it did have a bit of a learning curve. On the other hand, was you were familiar with it, it was fast to use compared to say...Shadowrun which routinely requires you to roll handfuls of d6s and count them. It also benefitted from uniformity and dice simplicity. Provided the person with the charts knew the system, all anyone else really needed to know was to roll d100 and add their relevant bonus. Compared to, for example, classic D&D in which a player might roll d20 to hit, and then if he's using a longsword he'll roll d8 and add his strength. Or if he's using a broadsword he'll roll d8+1 and add his strength. Or is he's using a flail he'll roll 2d4 plus his strength. Different dice and difference bonuses for each weapon. Plus a separate set of dice depending on the size category of the target. Two handed sword against a medium target is 3d6. Two handed sword against a large target is 1d10. Admittedly, D&D was an
awful system before they kicked out Gygax and started simplifying things.
Anyway, point being that a d100 system does not need to be complicated. In fact, d100 and d20 are basically the same thing. Rarely will you use modifiers that aren't multiple of 5%. Target of 65 with a 15% bonus on a d100 system is the same as a target of 13 with a +1 bonus on a d20 system. But percentages are generally more intuitive for people, the extra resolution is there if you want it for things like fumbles, and the wider range of rolls makes it easier to incorporate damage results into a single roll via a chart.