The structure of the extended endgame, inherited from near the very creation of Crawl, has some key design problems to it.
With all three problems side by side, it feels reasonable to try and repair the situation here. The proposal here addresses all three problems at once without cutting any content, and features discrete changes that can be implemented in testing segments.
Place only one of portals to Pandemonium or the Vestibule in per-game. The max number of unique runes gathered in a game shrinks from fifteen to eleven, with one instead of two demonic branch sets. Extended runs are shorter and have a clear variation point from one another, while thematic / mechanical overlap (such as Tartarus and Tomb or Cerebov's realm and Gehenna) is severely reduced. For stronger characters and experienced players it's a shorter slog and a shorter victory lap, for weaker characters there's no ease of powering up for Hells through doing Pan first, and for newer players it's more variety and wonder attached to not play both when learning and winning full-rune games.
One note, though: Pandemonium is generally held to be weaker than the Hells- the less elaborate vaultwork for the runes, the reliance on increased level spawns and the attrition of demonic rune vaults pale in the face of randomly devastating hell effects pressure. Some further buffs are probably warranted to the monster set (e.g. lorocyproca, blood saints) if it's going to take over Hell's presence in half of all games.
Since Pandemonium and the Hells have an uneven amount of runes, some more structural work has to be done beyond simply setting one to spawn and one to not, though.
The demonic rune is uniquely frustrating and awkward in the way it's hidden. Three choices, of the slim random chance in the vault on every floor, the spoiler-craving guaranteed recognizable guaranteed rune vaults, or the eventual landing in the additionally-spoiler-grasping holypan level, still outlines a spectrum of caution or reckless abandon but no matter the scenario requires going through many vaults and many floors of Pan to find it. There's not enough agency involved, no potential means of progress to the diving beyond choosing whether or not to take e.g. hellion_isle or holypan on a character when they show up.
It would be easy to argue that in the midst of this change to the greater branch that the demonic rune could be cut, but there are still positive qualities to the rune. It means directly engaging with something different from the specific themes of the other four Pan runes- the branch's set of mixed greater demons and classed demonspawn. It's a rune with high variability that mirrors its branch mixing together such a set of monsters, which works much more than, say, the out-of-branch threats for Swamp rune vaults. It also makes active use of a very large set of vaults and the constantly-iterated-upon random Pandemonium Lord designs, which would have next to no function with the removal of the demonic rune (beyond, say, forcing descending by placing most stairs inside the vaults, which introduces a different set of aggravations). Without the demonic rune, there's little point in the floors and contents in Pan outside of the other four Pan runes.
I've had drafts for a while now (see the Laplace branch) on an idea which tries to make it easier and meaner to find the demonic rune, which can be applied outside of this proposal: converting the guaranteed demonic rune vaults into announced serial vaults, and pairing up two per floor. Alongside a plotted-out second announced arrangement of four or five regular random demonic rune vaults placed on one floor, and with the option of increasing the chance for the serial vaults to spawn with each Pan rune obtained, this amplifies the choice dynamic- by making it much easier (with spoilers or not) to find the more dangerous demonic rune vault.
A further step in the above, which would be even more unusual but specifically provide a feeling of progress alongside more player control, would be to further increase the odds of the serial vaults (or even all of the Pan runes!) placing with each slain Pandemonium lord. Reducing bad luck through risk and high character strength seems to work for Abyss and is the principle of the above announced serial rune, anyway.
Keeping the demonic rune in a Hell-Pan roulette means an uneven rune count, though. There's a neglected monster set which can be put to work for this- the Holy monsters, currently mostly regulated to one vault in Pan and a lot of cameo / summon appearances. With constant reductions over the versions and years since the first big push to implement them contained many finnicky mechanics, Holypan has the two reliably functional and oldest angels alongside many straggler monsters not yet cut- this seems wasteful.
A quick-and-dirty single-floor branch draft is listed over in Purgatory. A single-floor subvaulted branch placed in the hells, after a long further set of changes and buffs to the monster set, provides one last nasty encounter and a severe shift in monster threats before finishing the Hells. The proposal for this branch could also use work, though- feedback is desired there, too.
Some aspects of these approaches held by others are orthogonal to the above Hell-Pan roulette proposal, and they're usually of much last drastic changes. They could probably get pages of their own if others wish to defend and expand on the rationales / ramifications involved, but the aforementioned problems of extended and the segmented manner solutions can come in mean it's worth mentioning the general gist of side-proposals for further approaches in abbreviating extended. Feel free to fit in feedback, arguments, and further proposals here, with signatures.