I'll cut right to it.
I know that Guildleves and all of that were supposed to be like an FoV or just a supplement to 'leveling.' What I propose here is to drop the charade and make leves (a great system BTW, especially how it's presented in the intro FMV) the primary leveling system. If grouping is inefficient and soloing is painful then...
Here is a (wish)list of things I personally would like to see in the revamped guildleve system:
1. Make hundreds of more leves* (quests, see below).
As it stands there just aren't enough. At level 22 on my Pugilist I've done the same leves almost 100's of times; they are just too repetitive.
2. Make leves more involved with the world.
Leves are too vague and don't reflect anything from the world or even the main story. As of now there is no uniting purpose to leves, other than haphazard quests thrown together.
3. Make the rewards from leves more substantial.
I'd want to see armor/weapons/crafting sets available, especially after a difficult leve.
4. Make some (optional) leves with better gear rewards, force you to group.
As it stands there is no 'reason' to group. If you want that awesome shield or sword you'll have to group up.
5. Make leves linear and involved in questlines.
6. Make leves the primary source of leveling.
Meaning you can level solely on leves. Once you're done with leves at a certain Aetheryte then you logically move on to the next one.
7. Make more Aetheryte camps.
This will provide a sense of progression and variety.
8. Remove the leve time limit and leve restriction.
Since these will now be quests, there will be no need for the restrictions.
9. Make leves and the completion of them give SP and EXP.
*So, how do I think this will all come together?
You'd go to the main city and pick up 1 leve. That leve would contain a series of quests that would end up with a mini-boss. That boss (in earlier levels) would be soloable. (I don't want to force grouping if I don't have to)
After that boss is killed (say, an Imperial spy) you'd receive quest reward, a class specific item, then you'd pick up another leve and head out from your previous camp.
After each questline you'd recieve a lot of SP and EXP, making it the fastest way to level. Each camp would hold 5-10 leves, but with 5-10 quests attached to each leve in a quest line, as mentioned above.
Grinding would be then reserved for mat farming.