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If they have enough strength to advance to Kursk, sweep south east towards Belgorod and cut off the Russian front line and attack from behind, possibly Ukraine could advance from the border, and have Russia retreat from areas near Kharkiv.
It’s possible that once the line has been broken a Wagner rebellion size force (maybe 10-20k + armour and air support) could steam roll through a whole swathe of Russia.
It's unbelievably risky for Ukraine, but its a stunning move thats gone well so far. Beyond the obvious success on the battlefield (currently), they've put themselves back on the top of the global news agenda, boosted morale and publicly given Putin a bloody nose.
I have no idea how they're going to get themselves out of it though. Unless they've been hiding a load of troops somewhere.