One thing that really seems to sit wrong with some players in older versions of D&D are the armor and weapons limitations on classes. Some players really want their magic-users to wield swords and wear armor. While I’ve never felt this way, I did come up with a system back in the late 1970s that allows any character class to wear any type of armor while doing a fair job of maintaining the “balance” of each class.
Base Armor Class: Each class has a base armor class that is in effect anytime the character is conscious and not tied up to the point they can’t move at all. This base armor class takes into account the character’s combat training which allows him to dodge and parry blows.
Fighting Man — Base AC of 5
Paladin/Ranger/Monk — Base AC of 6
Cleric/Druid/Bard — Base AC of 7
Thief/Assassin — Base AC of 8
Magic-User/Illusionist — Base AC of 9
Any character who is unconscious or heavily restrained has a Base AC of 9. Other classes should be slotted in on the level of the character that makes the most sense. ONLY the fighting man should get a Base AC of 5, however. Other fighter classes/subclasses should come in on the Paladin/Ranger/Monk line at best. The Monk is a special case, the AC by levels given in the monk class chart simply need to be replaced, starting with AC 6 instead of AC 9.
Armor: Armor adds to the character’s Base AC when worn. Armor may have side effects for some classes. (Remember that a plus to AC in older versions of D&D reduced one’s AC.)
Leather Armor: +1 to AC. Magic-Users and Illusionists cannot cast their highest level of spells known while wearing Leather Armor.
Chainmail Armor: +2 to AC. Magic-Users and Illusionists cannot cast their two highest levels of spells known while wearing Chainmail. Thief abilities are halved while wearing Chainmail.
Plate Armor: +3 to AC. Magic-Users and Illusionists cannot cast their three highest levels of spells known while wearing Plate Armor. Thief abilities are unusable while wearing Plate Armor.
Shield: +1 to AC, only when character is concious and mobile. Magic-Users and Illusionists cannot cast their highest level of spells known using a shield — if they are using a shield and armor tthe shield adds 1 to the levels of spells they cannot use.
Examples: An unarmored OD&D fighting man is AC 5. The same fighting man in plate armor and using a shield would be AC 1.
An unarmored 10th level (OD&D) wizard would be AC 9 and could spells normally. If that tenth level wizard wears chainmail, she would be AC 7 but would not be able to cast any of her 4th or 5th level spells. A 1st through 4th level magic user wearing chainmail would not be able to cast any spells at all.
This system was playtested with OD&D and AD&D 1e rules (reduce base AC by 1 as the worst AC in AD&D is 10 instead of 9) in the late 1970s and worked well. I did not use this much back then and probably would not use it today, but a number of groups in South Texas were using these rules back in the day as they were published in a local gaming club newsletter.