I kinda like bucket of dice systems, even though I don't much like needing to roll dozens of dice.
But such a system gets you scaled levels of success, and you can also get some reliablity in your skill.
D20+modifiers often feels too random, but smaller dice tend to have insufficient variance.
I kinda get that some people like "roll under" systems, but I think it makes modifiers often a bit more complicated than I like, and once you want to have things like success levels and the like, it has no real benefit anymore.
Personally, I also dislike penalties. I think you should only ever add. Subtraction is hard. Positive modifiers add to your skill roll, negatives add to your difficulty.
But two more outlandish systems I thought about playing with once:
"Everything goes to 12" (or 20?):
A rating like your attribute or skill training level is translated into dice:
The dice system uses something like d12, 1d1d0+2, 1d8+4, 1d6+6, 1d4+8, 1d2+10. (or d20, d12+8, d10+10, d8+12, d6+14 etc.) , So each of these dice can yield a maximum of 12, but the smaller the die, the higher your fixed value. A high rating means you roll a "small" die with a large fixed value.
The idea is that being good at something makes you very reliable, but you might still succeed on blind luck. The fixed value also gives you an idea of what might genuinely be a "routine" task for someone trained, but could still be a real risk for someone without that level of training and talent.
You might usually be called to roll two or three dice, for example attribute die, skill die and maybe specialization/gear die. Favorable additional modifiers come as extra die you can sub into the ones you already have (before rolling or after rolling is still to be decided?)
A expert climber with Body 1d8+4 and 1d6+6 without any special gear would roll 1d8+1d6+10, meaning anything of 12 or less is a routine task, while someone without the training but similar fitness would roll 1d12+1d8+4, meaning his chance is like 16 % or something to fail a routine task.
DCs for "doable" things would range from 4 to 32, but there are basically fundamental limits characters can reach. But I guess.. is that really useful to have? Do we need to have upper limits, does this in any way prevent power creep or power gaming?
Bucket of Dice, but you add all the dice up
You roll a pool of d6, and add the total, compare against a fixed DC. For every 10 points above the DC, you get an extra success level.
In a typical pool system, any die not a sucess simply doesn't count, in this system, they do. Making the success stage up at 10 is mostly for ease of use - Figuring out if you're 10, 20, or 30 above the DC is trivial (maybe even easier than adding the values of a bucket of dice in your head, so it might not be all sunshine, too.)