Cell is made from the DNA of Goku, Vegeta, Piccolo, Frieza, King Cold, and other living things. Additionally, he has access to knowledge of all of their most powerful techniques as well as those of the other Z fighters and villains they have faced. Upon absorbing the Androids (Cyborgs) 17 and 18 he has access to their Limitless Stamina devices. In the Funimation Dub Cell is made from the DNA of all the Z Fighters, including Tien, Chioatsu, Krillin, and Yamcha. This may also include Yajirobe, Korin, Kami, and even Mr. Popo (if Mr. Popo even has Cells or DNA to take samples of).
As part Frost Demon (Friezas Species) Cell is able to survive any mortal wound or condition so long as the nucleus within his head remains intact, due to Goku obliterating Cells' upper half with a Kamehameha it is likely that Cell is able to move this nucleus very quickly throughout his body. He is also able to survive in deep space, and goes through transformations, although his are not suppressed forms, but increasing forms. He should also be able to train himself into a new form, either as Coolers fifth form, or as Friezas' Golden form, or possibly both or another, becoming so powerful he creates a new form to hold the power.
As part Namekian (Piccolo and Kamis Species) Cell is able to regenerate lost limbs, in combination with the Frost Demons' Nucleus this means an ability to regenerate a lost body. He should also be able to sustain himself indefinitely on only water alone, as Namekians do not need to eat otherwise. He is shown to be able to spawn minions as a Dragon Clan Namekian does, as King Piccolo and Guru and Lord Slug have, he should be able to manipulate their biology as King Piccolo and Lord Slug did. Cell should also be able to use the Stretch Limbs ability of the Namekians, and even obtain a Giant form and a Super Giant form. Cell should even be capable of producing Dragon Balls.
As part Saiyan (Goku and Vegetas Species) Cell is able to gain power each time he nears death, as in a Saiyans Zenkai power, he uses this to obtain the Super Perfect Cell form, also as a Saiyan he obtains a Super Saiyan yellow aura and then a Super Saiyan 2 Electric yellow aura. He should therefore be equally capable of a Super Saiyan 3 type yellow aura, or even a Legendary Super Saiyan type green aura. In fact, with his tail and Saiyan DNA he should be able to use Blutz Waves to achieve an Oozaru form, for which cutting off his tail would do nothing as he regenerates. With the help of five other Saiyans, or possibly just five Cell Juniors with their own Saiyan DNA he could transform into a Super Saiyan God or even a Super Saiyan God Super Saiyan.
Bringing GT into this, Cell could achieve a Golden-Oozaru form and a Super Saiyan 4 form, he could have become 'Zetta' by Absorbing Super Android 17, so in addition to his Limitless Stamina Devices he could also have Hit-Absorbers and Ki-Absorption Devices.
As part Three-Eyed People (Tiens Species) Cell should be able to use Tiens Chameleon Eyes technique and his Four Arms technique, which would be crazy in combination with his Namekian Stretched Limbs and Multi Form.
Therefore, with all of this in mind, we could have a Super Giant Perfect Golden Legendary ‘Zetta’ Super Saiyan 4 God Super Saiyan 4 Golden-Oozaru Cell… with four stretchy limbs, hit-absorbers, and ki-absorption devices… who can survive in deep space on water alone. BUT THIS IS STILL NOT THE ULTIMATE CELL.
With the help of a Supreme Kai he could achieve a Mystic Form. The two would go on to create an egg that looks just like the egg that Cell came to the current timeline in. The egg is created using additional collected samples from: Broly, Bojack and crew, Omega Shenron, Hildegarn, Janemba, Hatchiyack, Vegito, Gogeta, Pan, Bulma, Pikkon, Dabura, Majin Buu, Majuub, Beerus, and Whis. Another Super Giant Perfect Golden Legendary Mystic ‘Zetta’ Super Saiyan 4 God Super Saiyan 4 Golden-Oozaru Cell arrives and from a different timeline universe, the two use the Potara Earrings from the Supreme Kai to fuse, and then absorb the egg.
And thus is the Ultimate Cell born.
Potara Fused Super Giant Perfect Golden Legendary Mystic ‘Zetta’ Super Saiyan 4 God Super Saiyan 4 Golden-Oozaru Cell… with four stretchy limbs, hit-absorbers, and ki-absorption devices… who can survive in deep space on water alone… and can use Broly Chest Bumps and Bulmas Genius!
As well as magical manifestation, creation, intangibility, portal-opening, material re-manifestation, dragon clan techniques, petrifying spit, candy-beams, unlimited regeneration from vaporization, God-Ki, etc. This comes at a cost of his handsome, handsome face which turns bug-like or rather like a demonic Frieza/Piccolo.