Upper_Krust
Legend
Hello AbdulAlhazred! 
My main point is that a DM won't have enough monster resources to fill 90 encounters over the course of the Epic tier.
So, the subsequent point is that you can't operate under a single (or even double) theme over the course of that many encounters.
So a DM has to either...
A. Design, Re-skin or Template potentially 90% of their monsters.
...or...
B. Have multiple themes and shorter adventures (even if they are linked under one larger 'umbrella').
If you look at E1-E3 (which runs two concurrent temes of Undead and Demons*), in every module, I estimate...
- 50% of the stat-blocks are new monsters created for that adventure.
- 30% are existing stat-blocks from soucres like the Monster Manuals
- 20% are modified stat-blocks or NPCs
*Arguably the two best supported themes in the Monster Manuals at the epic tier.
In fact lets just go over Death's Reach...
#1: New Monster
#2: Modified Monster, New Monster, Existing Monster
#3: Modified Monster, Existing Monster, NPC
#4: 2 Modified Monsters, New Monster
#5: Existing Monster, 2 New Monsters
#6: New Monster, NPC, Existing Monster, Modified Monster
#7: 3 New Monsters
#8: New Monster, New Hazard
#9: 4 New Monsters
#10: New Monster, NPC
#11: NPC, 2 New Monsters
#12: 2 New Monsters, 2 Existing Monsters
#13: 2 New Monsters, New Trap
#14: 2 New Monsters, Existing Monster
#15: 2 New Monsters, 2 Existing Monsters
#16: 2 Existing Monsters, 2 New Monsters
#17: 4 New Monsters
#18: 2 New Monsters, 1 Existing Monster
#19: New Monster, Existing Monster, Modified Monster
#20: 3 Existing Monsters (I think)
#21: New Monster, Modified Monster, Existing Monster
#22: 3 New Monsters, Existing Monster
#23: Modified Monster, 2 Existing Monsters
#24: 2 New Monsters, 2 New Traps
#25: New Monster, Existing Monster
#26: 4 New Traps
#27: 3 New Monsters, Existing Monster
#28: 3 New Traps
#29: NPC, 2 New Monsters, Existing Monster
93 Total Stat-blocks:
23 Existing Monsters (8 repeats)
13 Modified Monsters & NPCs
57 New Monsters/Traps/Hazards (14 repeats)
So about 15% Stat-blocks from existing sources, reused to make an extra 10% (Total 25%)
14% Modified Stat-blocks & NPCs
46% New Stat-blocks with an additional 15% gained from re-use (Total 61%).
So basically 15% of the content in Death's Reach already existed and 60% had to be created fresh. With the final 25% from reuse.
The question is where are you going to get your stat-blocks from.
Absolutely, but in essence then you are doing exactly what I am suggesting; having multiple themes and shorter (mini)adventures, simply linked into some overaching plot.

AbdulAlhazred said:Well, there is a LOT more to an epic story arc, potentially. It could go something like:
Each one of these phases can be reordered, interjected into the others, repeated, etc in a wide variety of ways. Entire sub-arcs can be created which recapitulate various elements of each phase in a narrower context, etc. The key parts are a fairly overall thematic linkage, epic scale, and epic action.
- Appearance of the threat. In the course of tying up whatever portion of the story happens in paragon tier the characters become aware of a greater threat. This threat might already be implicit in the setting, but it now becomes a matter that has direct relevance for the PCs. This phase further involves the acquisition of knowledge and information. This is good for at least 1 and possibly 2-3 adventures.
- Reconnaissance. Once some information has been gathered on the possible nature of the threat the party sets out to find answers to questions which require interacting with the threat in a fairly direct manner. This could be investigating remote locations, etc. The general theme being learning more directly and answering specific questions.
- Marshalling of Forces. McGuffins may need to be gathered or made, specific allies recruited, etc. The characters acquire the tools needed to defeat the threat.
- Direct Action. The characters take on the threat directly, ultimately leading to their victory or defeat.
My main point is that a DM won't have enough monster resources to fill 90 encounters over the course of the Epic tier.
So, the subsequent point is that you can't operate under a single (or even double) theme over the course of that many encounters.
So a DM has to either...
A. Design, Re-skin or Template potentially 90% of their monsters.
...or...
B. Have multiple themes and shorter adventures (even if they are linked under one larger 'umbrella').
If you look at E1-E3 (which runs two concurrent temes of Undead and Demons*), in every module, I estimate...
- 50% of the stat-blocks are new monsters created for that adventure.
- 30% are existing stat-blocks from soucres like the Monster Manuals
- 20% are modified stat-blocks or NPCs
*Arguably the two best supported themes in the Monster Manuals at the epic tier.
In fact lets just go over Death's Reach...
#1: New Monster
#2: Modified Monster, New Monster, Existing Monster
#3: Modified Monster, Existing Monster, NPC
#4: 2 Modified Monsters, New Monster
#5: Existing Monster, 2 New Monsters
#6: New Monster, NPC, Existing Monster, Modified Monster
#7: 3 New Monsters
#8: New Monster, New Hazard
#9: 4 New Monsters
#10: New Monster, NPC
#11: NPC, 2 New Monsters
#12: 2 New Monsters, 2 Existing Monsters
#13: 2 New Monsters, New Trap
#14: 2 New Monsters, Existing Monster
#15: 2 New Monsters, 2 Existing Monsters
#16: 2 Existing Monsters, 2 New Monsters
#17: 4 New Monsters
#18: 2 New Monsters, 1 Existing Monster
#19: New Monster, Existing Monster, Modified Monster
#20: 3 Existing Monsters (I think)
#21: New Monster, Modified Monster, Existing Monster
#22: 3 New Monsters, Existing Monster
#23: Modified Monster, 2 Existing Monsters
#24: 2 New Monsters, 2 New Traps
#25: New Monster, Existing Monster
#26: 4 New Traps
#27: 3 New Monsters, Existing Monster
#28: 3 New Traps
#29: NPC, 2 New Monsters, Existing Monster
93 Total Stat-blocks:
23 Existing Monsters (8 repeats)
13 Modified Monsters & NPCs
57 New Monsters/Traps/Hazards (14 repeats)
So about 15% Stat-blocks from existing sources, reused to make an extra 10% (Total 25%)
14% Modified Stat-blocks & NPCs
46% New Stat-blocks with an additional 15% gained from re-use (Total 61%).
So basically 15% of the content in Death's Reach already existed and 60% had to be created fresh. With the final 25% from reuse.
I can't imagine not being able to fill 10 levels with this kind of thing. It really shouldn't be that hard at all. I can also imagine condensing it all down to a single level if you were so inclined.
The question is where are you going to get your stat-blocks from.

Note too that while all elements will have some overall thematic linkage they don't require a narrow kind of tactical theme. The characters might need to engage the mighty Arch Fey in the course of seeking knowledge and information. This could involve various fetch quests, etc. Then they might need to seek a lost library in the Elemental Chaos (or some remote location in the world etc). Each of these can involve totally different kinds of opponents. The next phase might involve some travel and delving in order to learn more about the enemy (traveling into the far icy wastes of the north in an attempt to figure out where the legions of the Ice Lord come from). You get the idea. At each stage agents of the BBEG might show up now and then to harass the party.
Absolutely, but in essence then you are doing exactly what I am suggesting; having multiple themes and shorter (mini)adventures, simply linked into some overaching plot.