If you're saying drop stats then I've already explained what I think the problem with that is - people create builds which are heavily dependent on having high stats and for them to be arbitrarily dropped so you can be at the same level as an opponent could mean not being able to use weapons, armors, magics, hexes, or miracles that your entire build could center around.
For example, I like to use the Giant's Club, which requires 60 strength. If I got matched up with someone who's soul level was 100 and my strength dropped sub-60 as a result, I couldn't use that weapon. If I got matched with someone soul level 20, I couldn't use the Giant's Club, or my ultra Great swords, or my King's ultragreat sword.
If instead you're saying keep the stats the same but just completely neuter the damage, then comes the question of how do they neuter it and how much is reasonable? Getting summoned to fight a boss where you've got ultra good weapons that are doing a pitance of damage doesn't really seem like a good compromise at all either.
I understand your concerns, although my idea wouldn't allow for such scenarios. For example: Suppose I want to Co-op at FoFG and I get summoned by a character whose SL is 10. I'm at SL 300, so the procedure would be as follows:
Keep every stats untouched. DEX, STR, FTH, etc but soft-cap them all behind the curtains (multiply it by a proportionate percentage, like 0.09% when the breach between characters is abysmal) so that.
My current FTH deals between 1000 and 2000 damage with a single Sunlight Spear. If I get summoned by a low level character, by applying a soft cap to every single stat I have, I would be dealing (1000 ~ 2000) * 0.09 damage, which translates to 90 ~ 180.
If I have a +5 Santier's Spear that deals ~1300 damage for each combo, that would mean I would be dealing 1300 * 0.09 = 117 .
The percentage of downgrade should be proportionate to the difference between Soul Levels of the host and its summon. That would entirely balance the game. Everyone could use their crazy Hexes and Miracles but they would only deal less damage.
90% downgrade for a difference of 250 SL's and above.
80% downgrade for a difference of 220 SL's.
70% for 190.
60% for 140.
50% for 100.
40% for 70.
30% for 40.
20% for 20.
10% for 10.