The amount that the skills buff can be changed. We have traits added as we level that sometimes scale abilities. Sort of reverse traits can be added when we have stats scaled. So a Balance card might go from 10% to 3%.
A wider selection of tools doesn't really change much. The scaling should be based on your overall DPS or HPS. Also don't forget that leveling dungeons where sync occurs already need to account for a really wide range of party compositions. Synced dungeons aren't balanced. They just have the bar set low enough so that random parties can clear. With that being the case stat sync doesn't need to balanced to the 5th decimal place. Between levels 45 and 52 for example, WHM has the ability to build up a huge amount of AoE DPS with Holy. AST has nothing. WAR has AoE from the first dungeon, PLD has to wait until level 45. On top of all of this you have i level to further skew things. Stat scaling won't be as simple as just a flat potency change, but it doesn't sound particularly difficult either.
These can be scaled like buffs. Tanks could become softer with stat sync anyway and may need to use cooldowns more often to keep their defense as high as a lower leveled tank. Healing can already be easy depending on the dungeon and your gear anyway. Low Level AST is more or less pressing ED on CD since it's a full heal and damage is pretty forgiving for the more part.Then there's all the defensive tools tanks would have. They would need virtually no healing on bosses because the fights are so short they could just chain all their defensive abilities. And they would be even shorter than they are now for the above reasons.
The whole point is to find the scaling that keeps us from being OP. We can kind of try out the idea of stat sync now. Go into a low level dungeon unsynced with no gear and a level 1 weapon. I don't know how this stacks us vs each particular dungeon, but eventually as get weaker relative to enemies you'll be too weak to do anything no matter how many abilities you have.All this would just result in players chain pulling trash into bosses to make it interesting, because playing normally would be boring due to being op. I already experienced that in WoW due to very badly balanced content. It's hilarious at first but it gets old really fast. I don't miss it. I prefer playing content as it is intended to be played than to be so powerful that any stupid thing I press makes things fall over.