Theoretical Mirage Ultimates I'd like Super7 to do:

 As expected the Mirage celebration was a ReAction Repaint series. Disappointing, but predictable.

We kinda got a Mirage Casey. Also a weird 2k3/Mirage Hybrid Karai. So they won't be mentioned here:

Buuuut I'll try to make a decent Ultimates list:

Obviously the turtles are needed, but I won't mention them. 

Splinter:

New head, tan sleeved kimono.

For accessories:

Walking stick (w1 toon version)

Wakizashi

Empty Ooze canister

Art book

Shredder:

The only reusable bits here are the arms and legs...

New heads, new torso with sculpted tunic (reusable with foot ninja), new crotch new shoulder, wrist, and shin blades. New hands with dual blades on them.

For accessories:

Alternate Oroku Saki head

Katana

Thermite grenade

Toy Rack weapons

Foot Ninja:

Reusing the Shredder body with New shoulders, new forearmguards, the Foot soldier boots and new heads. New human Foot Ninja and with Bandana.

Accessories:

Shredder Rack weapons

Naginata

Pair of kunai

TMNT#2 April:

New heads (one Mirage Accurate hairdo the other being a better Toon head), New Torso and crotch, similar to w3 April, but slightly elongated to give April the waist she lost with her W3 figure and make her as tall as Ninja April. 

Accessories:

Hands 

2 Mirage Repaint Mousers 

Mousers:

A 10-pack of Mousers repainted in Mirage colors.

TMNT#11 April:

April based on the cover of TMNT#11

She'd be all new except the hands.

Neck peg should be compatible with TMNT#2 April (this is for the Deluxe repaints)

Accessories: 

Unmasked Casey head for Ultimates w10 Casey

Casey mask

Missing Casey hands

The Deluxe April figures EXPLAINED:

Since TMNT#11 April is Dark skinned and TMNT#2 April is light skinned, the Deluxe figures switch the skin tones. You prefer dark skin April get Ultimates TMNT#11 April and Delixe TMNT#2 April. If the opposite is true, then get Ultimates TMNT#2 April and Delixe TMNT#11.

Baxter:

I've done plenty of Mirage Baxter recipes. I still stand that his legs could be reused on Vernon.

Accessories:

2 Mousers

Mouser controller

Rextab building model.

Skonk:

The first non-variant on this list

He's mostly new parts but some could have potential reuse with Spuds

Accessories:

Rifle

Knife

Spuds:

Reusing Skonk's lower body, a new torso, head and arms.

For accessories:

Uzi

Sliced Uzi

I got to be honest here, the CRAP members are better suited for Deluxe figures than actual Ultimates even if they being "all new" betray the idea of Deluxe figures. BUT it's easier to sell obscure characters at a lower price than the Ultimates prices... not to mention that a $20 Upgrade pack would give me a chance to sneakily "upgrade the deluxe" to an "ultimate".

Renet:

She needs to have big boobs, this is Renet, afterall. Now she would require some Engineering tricks, because I want Renet to have her outfit glow in the dark, but I want her flesh parts being flesh colored.

The Prototype  April apparently was designed to have her skin be molded and attached to the molded yellow torso. So hear me out:

Using GITD Blue Plastic, as seem with Foot Soldier and Normal Plastic for her arms, legs, hooded head faceplate, and unhooded head

Hooded head, torso, crotch, plastic cape, hands slip on gloves, "crown", Time Scepter should be in blue GITD plastic. The boots, shoulder pauldrons, and knife should be in white GIYD plastic.

The clocks and details on her "crown" should be painted in GITD White (green glow) and gold paints. In order to have daylight color contrast and in the dark as well.

The faceplate for the hooded head, the unhooded head,  torso overlayarms, and legs should be in normal color plastic.

Savanti Romero:

He'd be Bebop Sized.

Two heads: one with an evil grin, the other screaming.

Obviously he's 100%new tooling. 

Colorwise, It's "Mirage inspired, but not Mirage Accurate. His pink flesh should be a grayish pink. The horns should be a mustardish color. His furry legs should be Toy April Brown hair colored. The hooves would be shiny dark blue.

His clothes, dark orange red with Glossy yellow for the accents.

Accessories:

Corrupted GITD Time Scepter in GITD Green

Renet's Time scepter in GITD Green

GITD blue and green Energy blasts that plug to Specialized hands

Energy swirls for the time scepter.

Rat King:

Repaint with a piece if wood as accessory is a bit lame, but accurate.

Bonus:

Naked April from The Birds The Bees and the Turtles:

I'll let you figure this one out.