Here's what i don't agree with you - while the rest i'm pretty much on the same wavelenght. Also i feel i need to explain better why i think paragon/caldesann are "useless".
D1/D2 had a long term objective - and it was gearing your character to the fullest, or reach level 99 (which wasn't required) and a lot of people kept running the game for that. It worked pretty well in my opinion. Also lots of other ARPGs have long term gameplay via other types of content (like Torchlight 2 or PoE maps) and i don't see them going bad either.
D3 has a long term content type - the GRs. However they lack some sort of purpose which isn't tied to "beat" something. All above examples didn't have nothing to do with this kind of challenge, you just played because your character got better via stats or loot - which is the core of the game imho.
The problem in current D3 is that character power is obtained way too fast and easy, and a soft cap can be reached in no time (while hard cap is a long journey). GRs are not the kind of content people want to run over and over simply because there's very few to gain from them - +5 mainstat or +1 gem level doesn't change much what you do. I'm considering baseline levels up to 800 because anyone with a very short playtime can reach that. I'mnot doing this because i'm done with journey even before i can reach that level (can you see my point?).
D3 is all around fast gameplay - the problem is that it dragged everything else with it. GRs for the sake of higher GRs is not something that appeals many people. Most people don't even complete the journey, which takes very little effort.
I can agree that ladder isn't everything. I agree that one wants more paragon and stats for getting more powerful and beating is own records - but even beating them what brings you apart personal satisfaction? It's completely fine for me, but i simply don't see the point.
I'll put down an example that i think i wrote already here oir on dfans (or both - i don't remeber).
Farming monsters of certain types (beasts, undead, demons) powers up an item you find via a cube-like quest (or slightly longer). Once the item is fully powered up, you can use it to open a portal to /summon a boss of that monster type. Once defeated, it can drop a specific item reward - it may be a cosmetic, some kind of ragent to craft specific items or a special legendary that "unlocks" a new build due to its power. The object power is depleted and you can start farming again for a leveled up boss version (will take more time to fill the bar).
The bar stays filled as xp does - it doesn't reset with session nor have timers or such.
I had also an idea on how drops would work (i had in mind cosmetic rewards only):different items are on different tiers (5 levels?); every level boosts the chances to drop the related tier item, reaching 100% when you reach the first level of the next tier (beat lvl5 boss and doesn't drop, lvl 6 drops the item 100% since you've "beaten" the tier) and goes one with the new items.
It may work the same for special legendaries, would be different with reagents until also reagents are tiered (1st tier drops X reagent for Y craft and so on).
This is the idea i have of long term content. Something i can play over and over and it's not tied to some sort of timer to beat. I can log in, farm for hours or 2 hours a week and i'll always progress towards a goal which is more tangible than a timestamp.
This would work also with seasons in mind; either you make them character-only (which seems just better themed for me) or make it account wide with seasonal characters having their progress reset and manage the whole thing like paragons (total xp will be added to non season).
Future patches with power creep can simply add more levels and more tiers.