I've been thinking this for a while as well.
The question is, what are monks 'designed' for?
I'd say out-of-combat healing. The reason monks are so abesolutely important is because they do heal in combat and enable you to push off anyone out of a lane.
So, this is what I think:
Create a new invisible debuff on demigods. 'In combat'. Everytime a demigod takes damage, they have this invisible debuff, and it lasts 1 second.
If you are out of combat, (don't have the debuff), priests heal just like they do now, perhaps even a little bit more. There is nothing fundamentally wrong with what they do out of combat and is really what monks are for.
If you are in combat, however, monks only heal 150 / 300 / 450 / 600 hp (Sedna's Wind II might increase this by 25 per tier, let's say). This isn't a 'huge drastic' nerf, but it's still going to be felt. (Level 1 Sedna I heal 160 without Blood, 240 with and the initial difference of 65 only gets larger as I buy items and gain levels).
I think that it's 'okay' for 150 to be greater than the 10% hp of a Level 1 Demigod such as Torchbearer (who is 1400 so the heal would throw up 140 out of combat and 150 in). This is such an insignificant difference that I don't think anyone cares.
Monks would remain still very effective early game, but you would begin to feel a need to upgrade them to keep them effective (at least, in combat, assuming you consider 450 hp to be 'effective' compared to the ~800-900 with Sedna Wind you would get normally). Plus, this would shift the balance of things a little:
- Angels become better, as they would harrass enemy generals, keeping the 'in combat' debuff on them, esp if they are just chilling, farming a lane.
- Ranged demigods get a needed buff (Queen, TB, Reg) and makes kiting more effective, esp against generals, since you can't completely ignore the kiter and just farm away and let the monks heal you.
- Most assassins have a good way of dealing with monks. UB has Spit (guarenteed in-combat debuff as it lasts 10 seconds) and Plague, Reg can snipe or kite, Rook has towers or arrows, TB has a ton of ranged stuff
- Sedna gets a slight nerf as Counter Healing becomes less effective (since Counter Healing is only active when in combat and they aren't healing as much. By level 8, Monks typically throw up heals in the 500-600s. Tier 2 would now be about 300 so it's not quite as good, but still good). Alternatively, Counter Healing could be buffed make the in-combat debuff last longer (maybe 2 seconds), or gives a 'in-combat' aura to enemy units regardless if they are taking damage or not.
- Queen shield gets buffed, you aren't taking damage -> no in-combat debuff
- Using mana to AoE creeps becomes a little better, since creeps would activate in-combat
- Sedna-Beast can't wtfuberrape towers at level 2 and walk away with full hp since they would heal marginally less (more like 80% hp
). Towers would still go down by level 4 if you left two demigods alone, though.
In general, this seems to keep in-touch with what Monks were designed for, still remain very effective during the early game, while making them not having quite the same impact mid game.