Hmm, I think the broader problem here is that for most RPG developers "something to do that's interesting" inevitably means (mandatory) combat. At least some of the classics very often (if not always) gave you the option, with the right approach, to potentially avoid combat if you so choose. In modern RPGs, with a few exceptions, that is less likely to be an option. Even if your game's combat system is phenomenal, you can only fight the same types of enemies so many times before it becomes dull.
I agree combats are the favorite fillers for RPG. But I don't think that most dev believe it's the most interesting filling.
The source of the problem is that in real time most players don't realize it's the same repeated at nauseam with only some minor variations sometimes. Have the action stress keeping them busy and generating some adrenalin is enough. So that's an efficient and easy filling once you have a combat system, then "design" a combat is just throw a pack of enemies so you can design plenty and even it's easy pretend they are random. It's much more difficult to design some puzzling, tricks, complex dialog relationship, secrets, stuff to find, more.
So most RPG that fill mainly with repetitive combats, use mostly only combats with no design but the pack of enemies thrown somewhere. There are exceptions like some Rip Bioware games where most combats have a specific tuned design. But usually combat fillers mean repetition and if you aren't sensible to the illusion of real time stress, it becomes very tedious very fast. A curse one me that grown with age.
For TB RPG the abuse is seen more easily more quickly, still a game like DOS1 classic has at least 50% of combats that are just repetitive fillers, so TB RPG doesn't avoid the problem, but to really work, TB combats need more design effort in each combat.
I like a nice mix of interesting dialogue, puzzles / side quests, and some challenging combat.
Again I agree, for me it changes all, I would add, secrets, stuff to find, hints/information to decipher and link to the context, details to notice (from item to grab to mid hidden path you could not notice), some interesting and diversified combats (challenging is a too restrictive aspect), alternate activities can be quite cool too, like a hunt, fishing pearls (Morrowind), or just fishing, figure the path to reach a destination, more.
I don't even need that those activities are complex, it's cool when some are complex, but it's more important to have many different things to do.
I'll pick a recent example, Battle Chasers Nightwar have all areas designed as dungeons and with a high level filling, many secrets, many stuff to find, many little hidden thing, many cool lore real stories (not some boredom history school book), many traps to be aware of and manage including many type of traps, enemies you could lure or sneak a bit and manage when and how you fight them, even a few sneaking possibilities (but as a tool for secondary purpose, not or not much for managing something with stealth). And so on.
Alas the excellent diversity is also because areas don't need be the terrain combat, so they can be a lot more dense with many obstacles and details. That aspect plus the excellent effort on filling and filling diversity is making one of the best filling I ever played, but the challenge level is low. I don't care about this last point, but i know some players will hate it.
I mean, it's not like I want RPGs to be "walking simulators" that simply have fewer enemies. I just want more CRPGs to give you multiple approaches; stealth, parley / deception , or fighting, rather than just fight after fight. Thus, I don't necessarily think there's so much a problem with "density", as much as lack of variety.
For filling and choices on how achieve an objective, I don't think there's a relation. Filling is about walking without to do anything. Filling is about having activities and have them diversified. Choice is only one type of activity, and a player tool to increase diversity himself. But it's not choices that will make a good filling. You could choose a combat or a stealth to solve a problem, but have a poor filling once you have decided.
About excessive filling, I can understand a few games games are going very far, with a filling some players can feel excessive. But that's extreme rarity, poor filling is a lot more frequent.
Also in reality filling can't be excessive if it wasn't players with some light mental troubles
which is named obsession of completion.
Completion is an illusion if there's any choices. You choose combat instead of stealth and you failed your completion goal, you wont know what was stealth in that part of the game.
Moreover completion is a bad management of psychology. Let non achieved stuff, unexplored areas, is to let open wide worlds and wide amount of possibilities, it keeps mystery to the RPG world, it keeps your imagination alive. At reverse exhaust all borders destroy this feeling and makes the world limited and mechanical.
Even more, I know many players don't have time to play, and consider replay as a waste. But with RPG and party building, and even more with choices, they choose a wrong genre. RPG are bound to be replayed. In that perspective let many activities for next plays is a smart move. So nope there's never excessive filling, and the bad DOS2 reviews I saw about that are non sense from someone not realizing he should consult to solve his minor mental trouble.
Now to be honest, completionist is certainly also an acquired taste, that learned lazy design to players. The lazy design is typically that the most important tool is hidden in the area and you get no clue on it, just chance or completion will allow find it. Such lazy design encouraged RPG players to learn to be completionists.
