This will be my last answer, as I believe continuing won't be worth, feel free to respond though. In top of that I don't think you have the knowledge and experience to actually bring a real discussion. Also what ima say is only my point of view, maybe greenfruit thinks differently.
So why it would be broken ? It would be broken simply by being stronger than other ultimates. The final damage has one drawback, it is delayed, nevertheless you can also increase your combo damage for 12 sec by up to 60%, you can easily get 3M bonus damage out of this only. And then ultimate can do up to 840xagi damage at once, which is already more than some ultimate, including swordsman (the one ima use to compare most of things). Give an ability to increase all damage by 60% AND then do 2184xagi (which would be highest of the game) damage is simply too strong for current game scaling, nothing else.
Greenfruit could reduce damage of ultimate to 225xagi and make it work with W, to not be broken, but I don't think it would be worth gameplay wise, and I feel it would be unfun. So in that context just have base damage of ultimate that increases itself damage is certainly best way to follow logic of reaper, be fun to play and easy to balance.
You seem to absolutely want to be single target damage so you can do more damage on main target, it doesn't work that way. 80% of AoE damage spell are more a QoL tool than real strenght of the character. Let's take swordsman, D Q(W) W(E) E(Q) and R are all aoe, but if greenfruit actually decided to make them single target damage, SM wouldn't be weaker at all. This would just be a quality of life tool that would be gone, simply less enjoyable if you prefer. Nevertheless the AoE of T and F can actually be helpful as it allows me to exist a bit against multiple foes for my ultimate combo; remove those 2 aoe would actually nerf me compared to the AoEs mentionnes above.
You wouldn't gain anything by getting ultimate damage be AoE because of the 12 sec delay, so use that as a "single target so should do more damage" is everything but a valid point. You are currently asking to have most broken ultimate of the game just because it is single target... That's not how balance works.
Also I would like to come on another important point, have strong ultimate isn't require to have strong character. The balance of the champions come from all spells, not only F. Swordsman's ultimate is probably the weakest of all in twrpg overall, and has 2m30 cd (part of the longest cd for ultimate). But it's not because F is actually weak, that SM is a weak character, I would even say that SM is actually top tier, not strongest but over average.
I'll come back to the "AoE" discussion, you say that assassin and reaper SHOULD be single target damage... Why ? What tells this ? The reaper's weapon is a fkin giantic scythe, how it would be senseless for it to hit many targets on a dash or on a spin ? (spoiler alert, Q spin and E dash are only aoe damage).
Same for assassin, why would an assassin need to be good to eliminate one target only ? I don't even think remembering any game were assassins were single target damage only. And even out of game, most of the things I see from movies, they usually assassinate many people at once. You just brought a fact that assassins and reapers of all games should be single target damage because it's their meaning... in top of not having to follow "general rules" in your own game, this is simply wrong statement in general too. And I mean, be better at killing isolated target =/= single target damage only required, and this is the "rule" followed I've seen the most often on assassins in general.
BW T doesn't do 4M damage first of all. Unless you mean 4M damage on a random creep with 0 mr, this means it would do 600k damage on endgame boss only with 85% MR (or 800k on all other boss with 80% MR), which is 2 to 4 times less than your R on endgame, which also has lower cooldown and allows you to get T passive proc 80% of time. Also BW's T is more useful for the gather than for damage or for the aoe. This spell could literally do 0 damage and BW wouldn't even feel the nerf compared to when she was nerfed on Q. Reaper atm clearly outdamage BW without a single doubt, but she has some other tool as well as you mentionned. Same for her E, take damage she does on a 0% MR target doesn't count, please try to bring real numbers and not "false info" to go on your point. If BW was actually doing 8M with just E and T, she would have been nerfed already, BW is no way close to one shot MA with RTE combo no matter the gear.
"The staff of plague can do over 2m dmg over 8 sec with only 20sec cd, its aoe+gives slow effect. These are just some example that i tested, and i believe they are closer to being broken than reaper's ult."
Staff of plague isn't even fucking out nor craftable nor checkable, it doesn't exist yet. Please stop try lying when you bring suggestions, greenfruit knows map well enough to see those, it's pretty sad to do that. Like beside discredit all your point, it does nothing, this was horrible choice you did there.
To come back to reaper, you say she has no tankiness, but she has a lot of mobility (you can dodge stuff with that), some DR with T (which is up quite often) and the 100% dodge rate for same reason. You ain't a knight, but you can't act like reaper has no survability tool, you are also agi and can easily get 40-50k hp without lower your dps too much. I think you should try to lvl and gear lightening mage, and then you'll see what is a character with no survability survability tool. And the character is so hard to play efficiently than you'll come back to reaper and beg for no nerf.
About CCs, ima exclude the stun from F because unreliable, but beside that the kind of root from E can be more useful than stuns if used properly.
Reaper also has some decent AoE as you can clean multiple target on same position, not best AoE character but the aoe does exist. She can do pretty high burst also, same not best of the game but she would kinda stand on top tier on that, higher than swordsman. And about sustainable damage, I compared to SM and I did 21-22M with reaper against only 20-20.5M in a min with SM, which is my main and a character I play much better than reaper. It means that the difference would be even higher if a pro reaper actually plays it. If she can outburst and outdps a top tier character like SM, then clearly reaper isn't trash character as you seem to pretend it. (Also as you probably understood, I couldn't use ultimate at full potential as dummy is full hp.)
(I don't say reaper is better character, just that she does more damage and has higher potential damage, there is other factor to take in considertion to actually have a real idea of what is stronger and weaker)
And finally about sins T, it does like literally no damage by itself, it's only very interesting to couple with another spell, mostly W, which is single target damage. So no it's not a super aoe damage and aoe stun tool assassin have.
The stun doesn't come from T also, it comes from W as well.
I hope I was accurate enough in my answer, because as I said earlier it would be my last. Next time you wanna bring a suggestion, please try to be objective and not bring false fact just so you look like you're right, because this actually tilts me.
Bring bad ideas isn't bad, it's hard to be completely objective and includes enough factors to have an idea of if it's good or bad. But bring wrong points and lie about some test you didn't do just destroy the whole point of the suggestion.
the T i was referring was the really old one, didn't realize it's changed now
no i'm not lying, yes the 4m, 3m and 2m dmg was on alfon dummy, when i did that test i didn't realize there was boss dummy in capital prius
i have done retesting on prius boss dummy and indeed reaper's damage is quite high (previously it seems low because i was comparing to the 3-4m one). so yeah it's my fault
i do being objective, and no it's not false fact, just the wrong one.
and pardon me if it seems like i was trying to look right. no it's not. it was because i like reaper very much that i did all the testing and give some feedback on things that at that time i think was lacking. so pls don't be too quick to assume things.
nevertheless, thanks for your input and insight.
still it would be awesome if F as it is be connected to W tho