Can you be a Yonko and an Active Shichibukai? lol Not saying Doffy was capable but like can you?
But things Doffy didnt have that the Yonko have:
- Immense Psychical Strength (Guys a tank but he's not a freak of nature)
- Advanced Haki Mastery or utilization
- Sky Splitting Kings Disposition
- Poneglyphs (or the desire to acquire them) He's not closer to OP than anybody ya feel me?
- Enough Territory (Dressrosa not enough, yes he has massive influence but he cant call on people to help him fight like WB's allied crews, Big Mom's Marriage Allies, Kaido's 10,000 Smile fruit users etc)
- Enough military might within his crew to challenge the WG and Marines (See above) Like the Marines gathered over 10'000 of their officers to do battle with WB and his forces. Doffy wouldn't summon that kind of reaction and couldn't match the sheer number of troops WB/BM/Kaido can call on.
- FEATS, this is often never brought up but the dude simply doesn't have the resume of taking on High Tiered or Elite comp. Like Shanks clashes with WB and Kaido and Draws with Mihawk, Kaido's resume is crazy, BM been a freak of nature forever and took out an Elbaf Village as a kid, WB considered WSM, BB killed WB and Took on the Remnants of his crew. DD just doesn't have these kinds of names on his resume or insane actions like breaking into Impel Down, Punching Celestial Dragons etc. His name rings out but its not ringing often lol
- Ambition. And this is a big one. Doflamingo scanned the playing field of the NW and chose to navigate thru it without conflict with the Emperors. This decision alone is why he couldn't be but i figured I'd list all the other reasons. Doffy actively chose to play fake puppetmaster feeling secure in his position a level or two below the real monsters of the NW even doing dealings with them to maintain his position. If he had challenged them surely he would have taken losses but he may have gotten strong enough to one day beat them like Luffy and Kid are trying to do. His goals were simply different than your typical OP pirate, and this may be a part of why I personally loved his character it takes him directly out of the running for an Emperor seat.
That all said his crew is stronger than most give them credit for.