One of the shortcomings of classic D&D is the relative dearth of guidance and tools it provides for running wilderness adventures vs. dungeon crawls. In the dungeon, not only do we have random encounter tables, but rules and guidelines for stocking each room. There are tables for determining the contents of rooms - Empty, monster, trap, special, and/or treasure - and explanations and examples of each. There are wandering monster tables, which can be used not only for determining wandering monsters during play, but for stocking the dungeon during the referee's design/prep phase. There are keys of map symbols to inspire creative dungeon design, and pretty much every symbol represents something relatively small and discrete, with which the characters can interact - a statue, a pit, a secret door, a rock formation, a fireplace, etc.
In the wilderness, things aren't so simple. There are keys of map symbols, but they're mainly of the general terrain type, not specific, discrete features. You could interact with an individual tree in concrete, easily comprehensible terms, but how does one interact with the abstraction of a forest? You could key specific encounters, but even a relatively small hex scale, say 1 mile, covers a lot of territory, and an encounter keyed to a specific hex isn't necessarily "right there" in front of the characters when they enter the hex, as a dungeon encounter would be when they enter a new room. There are rules for getting lost, but being lost only really matters when you have somewhere specific you want to go. There are rules for foraging, which can affect the resource management aspects of the game. And of course there are the random encounter tables.
Conceptually, there's a lot that I like about the wilderness encounter tables in (take your pick) B/X and BECMI D&D. The nested tables - roll 1d8 to determine monster type, and 1d12 for the specific creature - is simple, allowing for more possible monster types without resorting to percentile dice or bell curves. The major problem with the charts is that it's all monsters, all the time.
In the dungeon, that's not a problem at all. All the other terrain and special features are included in the map and key, and you know exactly where they are. Wandering monsters just add an element of uncertainty and a cost for squandering the precious resource of time. In the wilderness, where there's very, very little detail relative to the size of the area, having only monsters in the random encounter charts can make wilderness adventuring seem like little more than roaming about aimlessly, looking for meaningless fights.
I'm sure that there are DMs out there who can consistently spin gold out of this random monster straw, and make the encounters generated with those tables interesting and meaningful. I confess that I'm not one of them. Random monster encounters get old really fast, and it's hard to think of fresh reasons and new twists while on the hot seat at the game table. True, there should be a lot more out in the fantasy wilderness than just this roiling soup of monsters waiting to bump into the PCs and rumble, but without random tables, those sorts of things either have to be placed in advance in each hex, which would be a ton of prep work, or else created on the spot at the referee's whim, which has far too much of a Quantum Ogre vibe for my taste. The party has to deal with that ravine, or thunderstorm, or haunted battlefield solely because of pure DM fiat: for whatever reason, you want them to face that particular situation, and shazam! there it is.
Alright, getting to the point at last. I want wilderness encounter tables that are more than just monsters. Other things that should be there:
Terrain features. Streams, springs, waterfalls, gullies, ravines, box canyons, sinkholes, mesas, cliffs, glaciers, ponds, small lakes, caves, quicksand, quaking bogs, fire-scarred land, oases, a hill in the middle of a grassland, a clearing in the woods, etc. Terrain can be a hazard to be avoided or overcome, a place where certain resources like food, water, or shelter might be found, an interesting backdrop to a monster encounter, or just descriptive detail to a wilderness trek. Spells and Steel has some tables for terrain that could either be used as-is or expanded to include more features.
Weather events. Thunderstorms, cloudbursts, sandstorms, blizzards, torrential rains, gales, tornadoes, unseasonable heat or cold. Hazards in their own right, or complications to another type of encounter.
Special. This could be just about anything you care to put on a line in a table. Ruins, statues, magical springs, enchanted apple trees, ancient battlefields, dead magic zones, petrified trolls, fairy rings, standing stones, dimensional vortexes, burial mounds, wishing wells...These could be helpful, dangerous, or just curiosities.
Some of these features lend themselves well to possible dungeon adventures, e.g. ruins and caves, not to mention the lairs of those randomly-rolled monsters. Many DMs will keep a supply of small dungeon maps at the ready for just such an occasion, and drop them seamlessly into the proceedings.
The question burning in my mind right now is whether some or all of these things should have their own tables, to be rolled separately, or whether the first tier of the wilderness encounter charts should be expanded to include "Terrain," "Weather," and "Special" entries in addition to the usual "Men," "Humanoid," "Dragon," and so on, and then appropriate subtables for each of those new categories added for each terrain type. I'm leaning toward the latter, with a couple of "Roll again twice" lines, so that you might get combinations. Say, a monster and a terrain feature - all kinds of tactical possibilities there. Or a monster and a special - Is the basilisk guarding the healing spring? Or two different monster types - Are the giant and the orcs allies, or in the middle of a fight, or negotiating over something when the PCs intrude?
Yes, I think I'll go with that all-encompassing chart model, despite the fact that it could conceivably produce awkward results like two terrains at once, or two weather events at the same time. Re-rolling inappropriate results is inelegant at best, but my feeling is that it's outweighed by the simplicity of having everything in the same hierarchy of tables.