Speaking from a 1e perspective here, but it's close enough for rock 'n' roll and a lot of the same elements apply:
1) We removed dual class way back, just use multi-class.
Same here.
2) Stat bonusses have a huge impact on the game in 2e. I'd give humans XP bonuses per level.
What we did instead was tone down some of the benefits non-Humans receive, ban some specific combinations (e.g. no Dwarf Mages here!), and otherwise say that if a character could be a class at all it could advance in that class as far as its luck and abilities could take it.
3) Nope. Rangers and Paladins are part of the warrior archetype, so they will get additional attacks at set levels. Both classes already receive spell casting, Paladins get special abilities, Rangers don't take penalties fighting 2 handed and get companions. As is, these classes already outshine the warrior with options.
Same here, except as our Rangers are still the 1e version (thus untainted by a certain Drow) we don't have to worry about companions etc.
4) I steal from 13th Age and let Thieves (only) have the die type increase when using daggers (or close quater weapons) from d4 to d6 for instance.
Hmmm...interesting. Never thought of this. Not sure I like it from a setting-consistency perspective, but somehow increasing Thieves' close-quarters damage certainly holds some appeal. I've tried giving Thieves a variant on 3e's flanking for a to-hit bonus and thus far it's been pretty much a full-on failure, so this idea has my attention.
5) 2e and older editions are distinctly different from later editions (3+) where every class had a combat role. Thieves were exploration and infiltration experts etc. and were better attacking from range.
Or not attacking at all unless combat came to them, and otherwise largely leaving combat to the warriors.
6) I moved spells like Light/ Read Magic/ Detect Magic/ Alarm/ Cantrip/ Detect Undead/ Mending/ Unseen Servant and Wizard Mark to list I call "Lesser Magics" that all wizards learn as apprentices. These can be cast once per day for free. It frees up available spells per day for casters to make more meaningful choices. You can do the same with Priests. But only those classes - Paladins, Rangers and Bards use the normal spells per day rules.
I can't get on board with this. Wizard Mark is useless, yes; I long ago downgraded that one to a cantrip. Mending, Read Magic - who cares? But Detect Magic, Light, and Unseen Servant can be hella useful; and giving more of those for free really powers up the mages. I also wouldn't want every mage to have all these spells (other than Read Magic); but rather have to find or acquire them if desired.
What I did do for all casters (and is thus, I suppose, my way of making casters perhaps too powerful) is do away with the endless annoyance of pre-memorization. If it's in your book (arcane) or on your list (divine) and you've got a slot of that level left, you can cast it; just like 3e Sorcerers which is where I stole the idea from. No "up-casting" - you have to use the proper level's slot and if you're out of a level's slots then you flat-out can't cast any more spells of that level today.
7) I prefer 2e Bards, I like the "jack of all trades but master of none" concept. They should have kept this concept for later editions. If you want, limit them to spells that are sound based, create a bard specific spell list (limited to schools of Illusion, enchantment charm and maybe alteration), limit them to the Druid spell list (1e flavour) or if you want to go the other direction, limit them to specific psionic disciplines and sciences.
I went the bolded route; and added in some other things that could reasonably be done by sonic manipulation, including a sonic damage spell to give them some offense. Yes it means both rebuilding the class from the ground up and creating a bespoke magic system that only they can use, but I think it's worth the effort.
