If I remove all of the extra magic systems from my old game – because I would like to avoid the mishmash of things that existed when I used AD&D, Rolemaster, Call of Cthulhu, and Stormbringer magical systems all side-by-side – I really remove the most radical changes I made to the AD&D system. But there are two things that I’d keep anyways, and two things that certainly improve my game that I’ll throw out here for general consumption – and that certainly count as radical change:
I’ve always run a game with a fair amount of psionics in it – call it my love of Bradley’s Darkover series and Kurtz’s Deryni series – and this way always seems to work a bit better for me regarding figuring out what somebody psionic ability/strength is. In my own personal OSR my only struggle with keeping this was the addition of a statistic – but that seemed even more elegant and of more utility than the “IWC” calculation of the Psionicist (a class that I used extensively) and having a way to measure psychic strength will likely continue to have some sort of utility as I play. Taking the mental bonus from Wisdom make sense and gives this stat a use for non-psychic characters so it’s not a dump stat, and it general it can pretty much sit in the background and be quietly ignored unless it needs to be dragged out for some reason (unlike, say, Charisma or Intelligence which tend to be very front and center when role-playing).
Power as a statistic was more problematic to retain. It’s former use was pretty much solely because I used a fair amount of Chasoium-engine magic systems for things and keeping it will leave me with a temptation to bring those back. But I used them for a solid decade or so playing AD&D, and I’ve certainly used them in the past sixteen years with my homebrew system. There are also some tweaks that I do want to put into place regarding mages and cantrips, and there has ever been a debate in D&D about the low-powered nature of low-level mages and the above seems to be a very nice compromise that also retain the essential Vancian nature of D&D arcane spells (rather than divine prayer). It has one other benefit that lets me address one of the potentially most broken pieces of the standard rules. The bonus to save vs. Death Magic is sort of a throw-away that I just thought of, but I really don’t mind giving characters another potential bonus against things that kill them instantly – and I don’t think any of my players will complain either. All-in-all, not just not another dump stat, but a statistic where players will be torn as to what to do about – and I am a total fan of that as a DM.
But both these also means that actually means that I changed some of the bonuses you get by virtue of having a high Wisdom.
I have sort of a comment here that I’ll probably flesh out later – but I find the complaints around the thief-class made in some quarters of the OSR to be absolutely ludicrous in light of a game that not only gives characters a statistic for Intelligence but for Wisdom was well. Characters and playing has always been abstracted and the idea that you couldn’t do the same thing for a set of skills that many or most players and DM’s could easily be expected to be clueless about (Find/Remove Traps is the one that always gets talked about) has never seemed that out of place to me.