Btw, some of the ideas to improve classes I found on various persistent worlds.
1. Ambush resting system and the lessened chance to trigger ambush based on a character classes (ranger, druid, barbarian has lesser chance than others). (source The Three Towns (3T) which is gone now afaik)
2. Limited resting to certain places (and usually with requirements as food) - is common choice to balance the casters on roleplaying persistent worlds. However in gameplay this affect noncasters exactly the same and increase a need for healing. That way or another the impact on gameplay and balance is positive.
3. Tracking system to allow rangers track. (Havent seen in practice but its definitely out there)
4. Thieves guild, Druid grove, Arcane guild, Warrior guild, various adventure guilds... class-only areas with vendors, quests etc. - is a great idea how to improve playing certain classes.
5. Class-specific items with restriction on an actual number of levels that cannot be used with UMD. Common technique how to improve some classes, however often produce unwanted results boosting classes to much and the items are already there and its hard to change them back. Actually, it should be well designed from the start as changing such items multiple times is always a bad practice. If you like this idea be sure you have enough experiences with item properties and item power to avoid this issue.
6. Class-specific crafting system. Aka, only ranger can skin creatures and tan/cure the hide.
7. Well designed monster type distribution can significally help playing a certain classes. Lots of undead gives big advantage to the paladins and clerics, lots of animals makes playing druid/ranger easier. Also plant creatures and oozes are creatures that druid has focus against but this is not standard rather than spell modification from my community patch project (which reflects the 3.5 rules). Neccessary to point out there isn't many plant and ooze creatures out there but you can workaround this with the barkskin method. Basically you choose suitable creature skin, apply barkskin visual effect and you have a "plant" creature.
8. Hidden rooms, treasuries. Great addition for the rogue class.
Other ways how to improve balance that I uses:
A. Instead of changing certain spells, change the spell list add some new spells: example: spike growth and barkskin to the ranger, remove curse to the paladin, gust of wind to druid.
B. Create new spells, I usually choosing such that are only available to one or two class as wizards/sorcerers/clerics doesn't need more choices imo. Example: Longstrider (druid/rang), Goodberry (druid), Magic stone (druid/cler), Shillelag (druid), Fire seeds (druid), Shout (bard) etc. The 3.5 SRD is great source for this.
C. Ditto with feats.
D. Ditto with skills, for example Survival is great skill that can be added which would improve the nature classes such as ranger, druid and barbarian.
However these changes usually needs hak packs, which might not be a possibility for everyone.