"Reveal stealthed champs"
Honestly, revealing is better than the opposite idea- that is would just either not hit them at all or just disappear. Skillshots of all kinds can hit invisible champs, mark need not be different.
"Untargetable during dash"
This one I agree with- it is incredibly annoying that there is no way to stop them from following up.
"Reveal bushes"
First of all, 'smart use of bushes' does not exist in ARAM. You sit in the bushes because there is nowhere else to be.
Secondly, everyone is in the bush already. It's ARAM- if you can't see them- they're in the bush. That's all there is to it.
"Are they in the bush?"
"Of course they're in the bush."
"Instant long-ranged true damage"
All damage is "instant". Even DoTs are "instant" they just happen over and over again. It's a skillshot that hurts when it hits you- that's what they do.
They do a "little" true damage. Doing almost anything is more damage than just throwing a snowball.
"Can't be blocked by shields"
Good! Lee Sin Q is only blocked by one of those things- why would Mark be less? It's a Summoner Spell! You're sacrificing something extremely important so you can have it! It quite literally is supposed to be better than regular abilities!