To give solo players an endgame with time, involvement and gear equivalent to grouped players endgame (time, involvement and gear), how about making crafting quests where you have to go through a series of quests and bosses designed to be challenging for a solo player (no grouping allowed in these quests).
Have them a series of quests and trials and bosses at the end of each series, which gives chances to drop a researchable item for a special crafted set piece, that is equivalent, but different, to set items found by grouped players in their dungeons.
Have several types of new sets to learn to make.
Each time you run through one of these quest series you get to go toe-to-toe with the end boss who might drop the item you need to research to learn how to make a particular set piece. Each series could focus on a part of the body - eg head item, or leg item, or shoulder item, or weapon, or offhand, and so on.
For example you run through a series of quests with an end boss to learn how to make the head piece for a set item. The boss at the end may or may not drop the piece you need to learn for a certain set. He has a chance to drop the research piece for all set items for that part of the body, but may or may not drop the research piece you are still needing. You might need to do these quests several times to get the research item you need for a particular set. Then you'd need to take time to research that piece.
This way the solo player can make gear equivalent to the endgame gear of other players who group, and they have had to invest time and effort to learn these pieces.
The time it takes should be such that only a real solo player will be able to go through this, so solo players will have their own path to an equivalent endgame with time and effort and reward for doing all this.
_________________________________________________________________________
|| ||| Consolidated post covering further discussion in this thread ||| ||
Here's a rough workup on how they could set up a series of quests to cover all crafting types (woodworker + blacksmith + clothier + enchanter + alchemist + provisioner) so they all go through about the same amount of quests to get new gear researched and new recipes/potions/enchantments learned. Perhaps make it so for provisioners/alchemists/enchanters to learn the new recipes/potions/enchantment several drops are needed before you can learn to make that recipe or enchantment, so that you need several items, making that type of crafter go through around the same number of quests with end bosses as the armour and weapon crafters.
Was thinking maybe split these quest series up into the basics (magicka/health/stamina) and each of those were a quest series with a boss at the end, and each of those end bosses would have a chance to drop something for that crafter to learn, whether it's a new recipe or a new piece of research to learn.
magicka quest series =
+ magicka type sets (clothier + blacksmith + woodworker)
+ magicka/magicka regen (alchemy + provisioning)
+ jewellery enchants (enchanter)
health quest series =
+ health type sets (clothier + blacksmith + woodworker)
+ health/health regen (alchemy + provisioning)
+ armour enchants (enchanter)
stamina quest series =
+ stamina type sets (clothier + blacksmith + woodworker)
+ stamina/stamina regen (alchemy + provisioning)
+ weapon enchants (enchanter)
_________________________________________________________________________
Also given some consideration to how ZOS might be able to ensure they can make the solo crafting quest series + endboss system challenging without being impossible for certain builds.
To balance quests and endboss for solo players (who might be good at healing but have no dps or defence, or might be good at tanking but have no dps or healing, or might be good at dps but have no healing or defence), they could add in an NPC who you choose to take with you through to the endboss. There could be a choice of three different NPC companions you could bring with you (one dps type, one healer type, one tank type). That way any solo player would be able to take some backup who is strong in the area they're weak in, so the whole thing is okay for any kind of player to achieve, with some effort.
This would be a lot of fun for a solo player, who could use these to not only bolster their weaknesses, but also mix the experience up each time they play through the quest series, taking a different companion for a different experience.
And on concerns on having "forced" solo content...
In this case the content I suggested was "forced" solo content in order to earn the rewards (chances at learning new crafting sets and recipes etc).
Having people steamroll through this in groups would trivialise the content, which brings up the point that people don't actually earn the rewards and don't "need" the good gear they'd be learning by doing these quests. But if the content was, as above, balanced for a solo player + NPC companion then it seems possible to ditch the NPC companion and bring a real player in its place. And if they wanted more than one friend in there for social purposes, maybe just allow players to do the content for fun, without the chance of rewards/researchable set item drops/recipe drops (in whatever form they all take).
Basically so that ZOS can make the quest series and endboss difficult but achievable for any solo player.