Nobody ever claims that Ashe and Sivir are the same because they do different things. One has vision and CC while the other has wave clear and a move speed steroid. If you want to make more unique marksmen and have a diverse bot lane that isn't just "who does the most damage today" then marksmen need to be given more utility and less damage. They should do different things that allow your team to win, with the added bonus of doing ranged DPS. Kog'Maw has his Q. Rather than making him just W to instant win if he has a team or auto lose if he doesn't he should fill the resistance shred niche more. Or Ezreal who admittedly does break something every season being a marksmen with a long range "auto" that is his Q. These abilities and themes should be embraced so that games are less about the strongest ADC and more about the right ADC.
Even Jhin was very popular last season when marksmen were bad because he's a utility carry with three CC spells and some damage, and people nut every time he says "HOW LOVELY!"
So making fun, useful, and healthy utility carries is absolutely possible, and it should be the norm. Xayah and Cait are two zone control marksmen, once with more range and long lasting control and one with short range and more escape for shorter control.
Making the next carry be "I built two and a half items I kill your team now thank you friends" is a bad idea. Utility carries also usually have a more stable winrate because they are useful even from behind, but more because they enable someone fed to win, not because they are over tuned.
Also utility carries like Ashe can still dominate with damage if you just reach late game so it's not like not designing them as hyper carries gimps their damage.
TLDR Make more utility carries and less hyper carries.