Playability of an 'epic' strategy game?

Completely unrelated to your design but taking some of the same ideas is the GIPF project.
The idea here is that there is a basic game (GIPF) and a series of companion games (TAMSK, DVONN, YINSH etc.), all of which can be played perfectly happily on their own.
However, there are also some pieces (known as Potentials) which can be introduced into the original game. These confer particular bonuses under various conditions (or none.) However - and assuming you both agree beforehand! - then your opponent can try to neutralise your use of the Potential by challenging you to a side-game - generally one of the other GIPF Project games according to which Potential you are trying to use. And if they win, they get to cancel your bonus in the original game.

So over one game of GIPF you might actually play several other games, each of which has an impact on the outcome of the original game.
It's a clever concept, but perhaps a little too ambitious (I have enough trouble with the individual games; putting them together is frankly terrifying!)

It captures the idea of an evolving game that is impacted by actions that occur outside of the original game.

[In a similarly fashion, I've been exploring a sort of "campaign mode" idea for Lost Cities; there are a bunch of special cards that confer bonuses and each player starts the "campaign" with some of them and has the chance to win some more by meeting certain criteria within subsequent games. But you can only use one special card during a game, and once a card has been used it is gone for the rest of the campaign - and the cards are also additional VP markers at the end, so if you use them, you are forfeiting future points. The idea was that since a game of Lost Cities can be quite short (and made even shorter using these cards!), a whole campaign could be conducted in an evening, or spread over several sessions, as long as you track who had what cards and which ones had been used.]

Scurra, you beat me to it! The GIPF project was my first thought, especially as I'm developing abstract strategy games I hope to link together somehow, and am considering ideas other than the "hub game" idea of GIPF.

But even thinking about this caused me to consider the multi-session aspect of the overall game. One of the challenges of playing a "full" game of Gipf, with all the other games thrown in, is keeping you GIPF board from being fatally jostled over the days or weeks it may take to play through. Keeping game notation would become critical.

The other analogy that came to mind was that of Necromunda, which was the game Mordehiem was based of of. Orgak brings up the good point that having multiple segments is a different thing than having multiple sessions, as normally multiple segments will fit into a given session. This is a good thing, allowing the game to be packed up between sessions. Umless youhave a dedicated gaming studio, of course. 8)

I personally really like the idea of an evolving game, and, years ago, developed a few games that took an epic amount of time to play (like the old-school wargames used to). But I sense you're doing something a bit different. You've revealed very little of your project's details, but here are my questions:

1.) What purpose does the multiple-session format serve? Is it to allow thoughtful downtime? Is there meaningful prep-work to be done between sessions or segments? In Necromunda, shuffling equipment around and working on your miniatures filled the downtime meaningfuly.

2.) Is the multiple-session format a function of length, or is it implied by the different nature of the elements, such as in Project Gipf? Gipf sub-games are very different animals than Gipf itself, and changing mindset before playing can only be helpful. Even if there was sufficient time to play all the games at once sitting, a break of a day or so would hardly be a bad thing, unless you're doing some sort of "Ironman GIPF".

3.) Are the sessions distinct in a predictable way? That is, can I play session 4 knowing that I'll need this or that result to be successful in session 5, which is a distinct challenge? If so, then your game could be modular, with the modules argeed to beforehand, thus allowing the players to dictate the length to some extent. Even a few simple scenarios, such as Full Scale War (modules 1-8), Blitz (Modules 1, 4, and 8), Pocket War (Modules 1, 2, 4, 6, 8), and so on.

In closing, I'd suggest formulating your idea as to what game objectives the format serves. Otherwise, it's a lot to ask, for few clear rewards over a shorter game.

I have played both Necromunda and Heroquest, which both have the same play sequence as you describe. The thing I found necromunda did well, is balance. Because as the characters level advance, you have to make sure no one falls behind. Or you might loose players much before you reach the +20 Sword of Armagedon. In Heroquest, we had decided to share the resources and equip in the best interest of the team, thus no one would fall behind.