Jacob Lewis
Ye Olde GM
I've always believed that the game had too many levels and too many spells. And too many spell levels.
Hmm.. thought I had more to say about that.
Hmm.. thought I had more to say about that.

Starting with goals of the design: what job do you want randomisation to do? Is it to prevent minimaxing? To challenge players with unpredictable combinations? To build tension around level-ups? To say something about the game world? To say something about the characters.
It would take a bit if design work, but were you going this route you might draw from designs of boardgames like Mage Knight. Each class has a pool (cards maybe). Draw N from pool and return N-1 to float. In later levels, take from float or draw N more from pool. Etc.
BUT You stated a goal as - make the world fit together. How does randomisation achieve your goal? Is the real issue character power? If it is character power, what are the dimensions of power that most matter? Is it number of feature? Hit Dice? Proficiency Bonus? Ability Scores?
I madeSorry I made.you type all that about the randomization. I edited the original post to try to make it clear that it's a slower and more granular skill/ability/power buying with xp (instead of big chunky levels), and not the randomness that I'm pondering. It just so happened.that GW used randomness. The randomness feels like it would be frustrating sometimes, but I have no recollection of it from the early 1980s when I last played it.
Edit: Although others might really like the randomization (see a post a few down from here).
Part of it for me is trying to picture what that 20th level BBEG is doing while the PCs head that way in level. If they're a spell caster, what can one do with teleportation and dominate person and all of the scrying spells... The concentration and attunement rules in 5e feel like they do a lot to help this. If the powerful bad NPCs and monsters are around without powerful good guys, then I wonder about why there are castles and armies. If there are powerful good guys, then I wonder about why they're sitting around waiting for the PCs. (I guess this is something the comic books deal with. If there is a silver-age Superman around, then your street level heroes are redundant. If it's a less omniscient superfast Thor at the top of the food chain, then there is a lot more for the street levle heroes).THe first thing, I think is that it really depends on which version of the game you are playing. For me, 4e has the most restricted evolution from level 1 to level 30, whereas AD&D has the lowest start and 3e the highest finish. 5e is somewhere in the middle, but a level 20 5e character is not a superhero. He has powerful abilities, but he is far from invincible and there are certainly foes who will challenge him. For me, the only difficulty comes in when you want to imagine his foes. Yes, in that paradigm, it has to be something on the level of the ruler of a plane or at least a powerful being, certainly not evil joe from around the corner. Now, it does not mean that the BBEG needs to confront the characters from level 1, there will be a whole pyramid of foes.
Which is why I've always thought about the idea of using E6 to try and "solve" these issues in the D&D space... but inevitably I always comes back to "Why am I bothering to try and jerry-rig D&D into something its not, when there are plenty of other RPGs out there that actually do what E6 is trying to get across?" And at that point I just accept that D&D is what it is, and let it be. Yeah, I might come up with the occasional house rule for certain things just to play the game a little differently... but any grand mechanical changes? I design them on my off-time thinking they might be this new breakthrough in how I run my game, but then once I'm finished I realize that D&D just doesn't lend itself to it. If I want grand mechanical changes? Just play a different RPG.
I sometimes think I want grittier. But then I picture the extra dice rolls and the things that wouldn't happen in D&D games I've loved if it was grittier... and I'm not sure... So I'm trying to think about what I can keep in D&D that deals with my world building power-range angst.And yet, they happen all the time in the genre, whether movies, books, tv shows. And no one complains, and people find them awesome.
And this is why these more gritty games do not model what's happening in the fantasy genre, in particular the high fantasy one. After that, it's merely a matter of taste.
I madefivesix [see edit] changes for my campaign
Maybe you can use, modify, (or simply ignore!) these sorts of ideas? I can explain the design thinking behind any you feel interested in.
- Ability Scores cap at 18
- Hit Dice cap at 6th
- Proficiency Bonus caps at 6th, except for uses of features between rests
- Spell levels cap at 3rd (fireballs etc, but no polymorph etc) - higher level spell slots are used to cast spells half their level, counting cast at their level for counter and dispel
- Short rests are 1 day, Long are 3. There is a 10 minute "Breather" to spend HD. You recover 1HD per sleep. Long rests don't restore HP.
[One other change! 6. Deck-based ability score generation.]
Roughly, characters jump in power as they complete each tier. Fighters for example go 1x, 2x, 3x, 4x attacks. Additionally, they scale continuously in power due to ASIs, PB and HD.I like it, and it feels like it catches a lot of the e6 feel. I'm still mentally stuck a bit in 3.5 (in spite of currently running two 5e games). There the trick is what to do with the feats as they advanced past that... and how to deal with multiclassing to go with the feats... and the fact that they still really advanced quickly compared to the rest of the world... Thinking in terms of 5e, what is the advancement like when they go past 6th level? (Do the attacks keep getting deadlier, while the hp stay the same?).