OP, I gotta ask: What exactly is your goal in this thread?
Like, are you looking for general ideas concerning tabletop games? Are you looking for specific ideas or criticisms on how to improve an existing game? Are you just trying to figure out how RPGs works? Is this just a general thread for meta?I've posted a lot here, but I'm getting the feeling like I haven't really addressed what you were looking for...
I'm going to just go down the list for things mentioned in your OP:>highest priority would be playable races and monsters
The highest priority for homebrew ponies would be making sure they're all of the same relative power level with eachother. I posted my ideas for Earth ponies; I'll post my ideas for unicorns if anybody asks.>Amazingly entire campaigns have been played without precise knowledge of creatures' strengths and weaknesses, but setting the definitions straight will help nuance and variety.
I don't think that's accurrate, tbh. We've only had a couple "campaigns" here, and by the looks of it most of the GMs involved had an idea of player stats. This is the kind of thing you decide before a game, and maybe retroactively fix if it's bad.>Campaigns set outside the typical high fantasy realm of DnD
I've said it before, and I'll say it again. It's probably more work than it's worth to try and hombrew all this stuff. D&D was made for medieval/classical fantasy, not modern warfare. You'll probably have a better time playing Shadowrun or something else than doing all that work to combine two concepts.>present further uncertainty
In more ways than you'd think.>Most guns
The 3.5e DMG says to refer to D20 modern for rules and stats concerning firearms. The stats there are usable.>vehicles function in combat?
Also in D20 Modern, but the bigger concern is how your characters will be able to use them, particularly if they'll be able to use their D&D abilities aboard a vehicle, and if the presence of said vehicles will impact the dungeoneering experience.
The elemental Vessels in Eberron are also worth looking at.>given it's a WW2 setting
Are you playing d&d, or are you paying a wWII sim? It's going to be a bit of extra effort to make "Dungeons And Dragons" look as expected in WWII level warfare is the focus of the adventures, especially since most PC classes do not equip PCs will the skills and abilities for such; unless of course your PCs built for that in particular.>Can bits be expected to buy the same things as the recommended valuation of GP?
50 gp is a pound of gold. A bit is probably closer to a copper at most, a lead bead at least. What you call the currency really doesn't matter, but imo it's much easier to simply use gold, as gold/silver/platinum pieces, as precious metals can't be created/fabricated as easily.>playable races and monsters
There are 5 monster manuals in 3.5e, and dozens of templates. Finding monsters is a non-issue. To reinvent human-shaped monsters, just make the quadrupedal, increase their movement speed, possibly swap out their natural weapons, and apply the Fingerless rules I posted before.
as for playable races, focus on giving them equal abilities. At-Will Flight on pegasi is a lot, especially if you intend to make ranged combat stronger. Make sure that Earth ponies and Unicorns get equally-valuable abilities, and make sure said abilities are specifically defined before players make their characters.>why don't we build a reference as a community project?
There are a lot websites dedicated to this. you should check them out.>What needs improving?
No comment on this for now.>unfortunately I don't have the book
You don't need the books, just know where to find the PDFs online. I already told you about TheTrove.>We've had some longstanding campaigns on /vx/
Ngl, I've participated in some of those games, but over the years I've come to the conclusion that chansites might not actually be a very good medium for good tabletop games, just imo... I'd recommend trying out other mediums; online d&d can be fun if it's organized properly.>there is no complete rulebook or conversion set that allows for an immersive experience
Well, Ponyfinder technically counts as "Complete" if you're looking for a place to start. I don't really like it though (then again i prefer 3.5e).
Also, your OP includes links from 5e, 3.5e, and pathfinder. If you want clear gameplay, you're going to have to be clear about what system you're using.