Decide full or mild hybrid!
You could read the patent to see how powersplit hybrids work:
It describes connection of the engine to planetary carrier, generator to sun gear, drive from the ring gear, return of power from the generator to a motor downstream of the transmission, and balancing of the generator load (square of velocity) so that it represents the vehicle's wind resistance in miniature and generator's inertia so that it represents the vehicle's mass in miniature to the same scale, and also having a gear ratio providing this scaling. With that set up, the hybrid self adjusts its gear ratio to different road speeds, when encountering hills, accelerating, etc.
There are other ways of making hybrids, eg the mild hybrids. Do a search on Belt Alternator Starters, (BAS).
Also, have a look for Integrated Motor Assist (IMA) to see how Honda do their hybrids.
Start by thinking what functionality you want:
start-stop, also called idle-stop or stop-and-go;
regenerative braking;
acceleration boost or gearchange smoothing.
For idle-stop, the motor-generator needs to mechanically connected to the engine to start it. For regenerative braking, the motor-generator needs to be mechanically connected to the wheels to recover the braking energy. So for complete hybrid functionality you either need two mo-gens, like the Toyota Prius, or one mo-gen and clutches to change its connectivity.
Once you've decided mild or full hybrid - and motor size is a grey scale, not the black and white, mild or full that I make it sound like - and decided on your base transmission architecture: normal CVT; conventional transmission or powersplit CVT, and decided where and when the motor is connected (the functionality question), I think you are well on your way.
If you want to do something original, you can mix the clutches and gears for a transmission with the
clutches (providing hybrid motor switching for different functionalities) and
gears for power merging to come up with your own original transmission design, eg use the transmission's clutches and gears so that only one mo-gen is required, and to vary the gearing on the motor at times so that it can be downsized.
I said that motor size, full or mild hybrid is not a black and white thing, more or a grey scale, but nevertheless, the question arises whether you want the vehicle to be able to run at times on just the electric motor.
Background info:
Honda Insight - about 80 g/km CO²
VW Lupo 3L TDI - about 80 g/km CO²
the Renault Clio 1.5 dCi (110g/km),
Citroen C3 1.4 HDI (110g/km),
Peugeot 206 1.4 HDI (113g/km),
Smart City-Coupe (113g/km),
Toyota Yaris D4-D (113g/km),
Toyota Prius hybrid petrol/electric (114g/km),