Allandaros said:
So I've been looking at the 3.5e handbooks recently (I thought the game I'm joining might have been going with 3.5e, but we went with 2e instead), and I've decided that I like the ideas of "taking levels" in a class, and the general concept of feats.
Can you guys give me any recommendation as to how to integrate these into 2e without causing problems? (And no, I'm not looking for the answer of "play 3.5e"

). I'm asking because I'm somewhat scared of the Knock On effect, and whether it would be feasible for these to be inserted into a game, but still have single-class characters be worthwhile.
We had some house rules in the 2e era that were quite similar to things that became standard in 3e. By the time 3e was announced, our own home-brewed version of 2e had the equilvalent of feats, something similar to 3e multiclassing, and level-based ability score improvements.
For feats, we were using the Player's Option rules (Skills & Powers, Spells & Magic and High Level Campaigns, but we weren't using the very complicated combat rules from Combat & Tactics). With each character getting 5 Character Points per level instead of the normal RAW progression of WP/NWP, those skill points could be spent on NWP's (i.e. skills), weapon proficiencies, or enhancing abilities with weapons (learning things equivalent to feats). We had long lists of techniques and abilities you could spend skill points to learn (a lot were out of High Level Campaigns, or inspired by them).
For multiclassing, we threw out pretty much all the racial limitations on multiclassing and what race could take what class. Whenever you had downtime for training, you could announce you were picking up a new class, assuming you had the ability score and alignment requirements and a teacher or other way to learn the class, and spent the amount of time the DM required in training/practice. You could only learn one class from each group: Warrior, Priest, Wizard and Rogue (and Psionicist, since our games had psionics in it). You could only actively progress in three classes at once, but at any time you could "bar" progression in a class, meaning you no longer acrue XP in the class and stop actively progressing. Once barred, a class was essentially done progressing forever, although we allowed a Wish, (or years of practice and training, effectively removing the character from the campaign) to return to a barred class.
In practice, it actually worked pretty well, although few people ever multiclassed to Mage because the DM was a huge stickler about how expensive and valuable spellbooks were (new blank spellbooks cost 2000 GP, and he made a big point about how the mage was the richest member of the party at 1st level because his book was so expensive and was a huge nitpicker for page counts in spellbooks, but NPC wizards when killed never seemed to have books on them, or on the rare occasions when they had one, they conveniently were full to the last page and had only spells the party wizards already knew). It was a very rare treat to actually get a spellbook, even a tattered old one with a few new spells and a handful of blank pages so somebody could try to learn a new spell.
As for ability score improvements, the 1 Every 4 Levels (for your highest level class in 2e terms) would work better than what we did. We had to have you learn a NWP called "Self Improvement" (4 Character Points), and every time you leveled up in your highest-level class, you rolled 5d10, and split the result up evenly as percentile to the prime requisite of all your classes you are actively progressing in. So a single classed fighter would roll 5d10, get 28 (for example) and be 28% of the way to another point of Strength (or another Percentile Strength category if they were at 18). If they were a Ranger/Enchanter/Psionicist they'd have to split it between Strength, Dexterity, Wisdom (for Ranger), Intelligence, Charisma (for Enchanter) and Constitution (for Psionicist), all 6 scores, so each one would only get 4.6% of a point into each.