It's not that hard to approach this from the other direction: start with the base* 1e chassis then strip out the complicated bits (e.g. weapon vs armour type and weapon speed), re-do initiative into something much simpler, pick one multiclassing system and have it apply to everyone, and make a few other minor tweaks e.g. removing gender-based stat modifiers and relaxing or eliminating demihuman level and-or class limits.
* - as in, pre-UA; be vee-ee-ee-ry careful when considering what (if anything) to add in from supplements released 1985 or later.
I mean, sure. The 'completely different games' bit is anti-Arneson-lawsuit boilerplate, so you can add stuff on to B/BX/BE-parts-of-BECMI or pare stuff down from AD&D and end up within spitting distance of each other. The differences you end up with will be trivialities like whether the reaction table is 2d6 or %-based.
They're not perfect, but at the same time not as bad as they're made out to be. Giving them access to a slightly wider weapon selection and-or allowing them to use some previously-Fighter-only magic items can make a huge difference.
To each their own, but hard disagree. TSR-era thieves are flat out bad. For the grand benefits of their class specific abilities and slightly faster level advancement*, they end up having terrible hp, AC, saves, net damage output, upper-level utility, post-name-level followers, and eventually even bad added-later qualities like nonweapon proficiency allotments.
*those extra 1/2-2 levels not meaning, in general, that they are ahead on the numeric qualities used in-game like hp, saves, or attack to-hits.
Their specific abilities do let them get some significant spotlight time, but also tend to draw great big targets on them. Finding/disarming traps puts you right in said trap's way, as does opening locks when the trap is on a locked door or chest. Sneaking off to scout also leaves them isolated and vulnerable whenever the stealth fails. Backstab enhances their damage output*, but only highlights how low their otherwise extant combat output really is (between lack of fighter Strength, to-hit, and durability-based ability to stay in the front lines of combat).
*I will not get into the existing discussion about how frequently it could be used, except to say that 'no more than every other round' was well and above anything I ever saw happen (it was usually closer to 'every other fight')
More to the point, (especially with AD&D) actually being able to use your thief abilities was something the ruleset disincentivized, and eventually obviated. The DMG was not shy about indicating that a thief was a fool if they thought they should try climbing walls that were slick or damp, could move while hiding, or hear a noise while wearing a helmet. Wall-climbing checks were per-round, making even high-90%s climb scores an apt lesson in 1-(1-P)^N failure chances. Every opportunity to make the activity not-work (or even be harmful in the attempt) was realized, in a way that generally isn't true for magic or even fighterly pursuits. Most notably (at the end of the day), a thief hitting name level was still only becoming somewhat reliable at the same tasks they were attempting at 1st level, despite spells and magic items having taken over those roles (if they were used at all, given the supposed movement out of the dungeons at around level 4) a long time ago.
Regardless, I also really don't think giving them a few new weapons or magic items will really change the calculation much. oD&D-BECMI thieves could use any weapons (and through that, most fighter magic items), and it did not make them significantly better.
Yes, I'm sure everyone has had a thief that they played and loved*, and even thrived playing. I have too. However, I feel that that is in spite of the specific game rules for the class, not because of (or just supported by) them.
*the flavor/theme of a thief character certainly is irresistible at times
This is an exaggeration. Being a level or two behind is pretty equivalent in all editions. More than a level or two definitely gets bad in 3e-5e, but in the TSR editions if you were more than that behind you were generally hiding in the back anyway. The HP difference mattered and sometimes plain level/HD (see stuff like Death Spell, or Dragonfear in 2E).
What the old XP charts +XP for treasure were better for was getting new low level characters caught up faster.
I'm going to tentatively agree, in that I think the notion is oversold in 3e-5e. In most all D&Ds it can be real bad to be several levels behind (dragon's breath alone makes this practically a truism), while at the same time you always are playing with characters with vastly different durability and vulnerabilities. 5e possibly is better than most in that there are fewer save-or-die effects, and the specifics of post-hitting-0hp. And yes, the big advantage of TSR D&D is the doubling xp requirements through name level means the new level 1 party member can stay in the back and level up to your level X by the time you're ready to advance to level X+1 (some restrictions apply*).
*only one level per session, has to still participate, may be penalized if hanging in back qualifies as not fulfilling class role, see DMG for details, etc. etc. etc.
Yup. The original Thief design was super fragile and incompetent, and TSR never gave good instructions to the DM on how to run the game in a way to make them useful and functional. The opposite, sadly. Gary kicking them while they were down over and over again in his advice in the 1E DMG (and by making Find Trap a skill in the PH), and 2E being not much better. Allowing point distribution by player choice let the Thief start out decent at a couple of skills, but their AC, HP, and Saves remained bad even with the faster advancement factored in, and the restrictions on Backstab meant that if the DM adjudicated it by the rules you'd get it at best once a fight, and that's assuming the DM never used monsters which couldn't be backstabbed. Which would be a weird campaign indeed given how many monsters were immune.
I am still a bit torn, philosophically, on the point of the Thief. I've played them and seen people play them in old school, but they always need some help. I'm not 100% convinced that turning them into a reliable "Striker" as WotC did was the best course of action, but I do think it's more in the spirit of the original 1974 rules than the actual TSR Thief implementation was.
If I were in charge and it were 1975, I would do something along these lines: Make most thief abilities (the stealth, climbing, and trap-finding ones) explicitly something anyone could do* -- some if and only if you were wearing certain levels of armor.
*either describe how you do it, or use level-based adjudication method, as the group prefers.
I might also make a specific thief class who got the rest (probably lockpicking, read language, and using cleric/MU magic items), along with some other abilities*. This separate class would not get D4 hd, 3rd best attack, worst saves, etc. Instead, it would be roughly identical to the fighter (or maybe cleric), except no plate armor (maybe no chain) and perhaps some decrease in offensive output**. Honestly, what this alt-thief could do that others can't (especially in a game where everyone can hide and climb) is limited enough that the reduced staying power of worse armor is sufficient to keep them equal in to-group-value to a regular fighter.
*particularly some that became useful right as spells which obviated existing thief abilities became low-value and/or dungeon-crawling declined as an activity
**be that no 2H weapons or no fighter 18/## strength, or introduce weapon specialization and they don't get it
I've always seen the thief's backstab as a skirmish implementation of the Voltigeur. A Napoleonic term for light cavalry units that charge file & rank units of the enemy transversally instead of frontally. Very devastating.
Skirmishing, area-denial, suppression fire, keeping your opposition focused (or just unable to ignore) one unit to the detriment of focusing on another -- there are a lot of really important IRL combat endeavors and tactics that some or all RPGs (and even wargames) sometimes have a hard time emulating.
I like the
concept that all the 'mobile/tactical' D&D characters (TSR thieves, WotC rogues, 2nd edition+ rangers, monks, etc.) are doing this kind of thing. I'm not sure that they really are working in the same way based on the ways a typical D&D scenario isn't a battlefield or warzone. I guess if (say) a dungeon's opposition force knows where the main PC group is -- and could send all their armored troops at them -- but instead keep 10-20% of them back because they know there's a thief stealthing around the corridors who might backstab the dungeon's mages or such, then it works on a similar principle. I just think that's kind of a niche scenario in a D&D game.