I had some thoughts on this last night. I really have no idea about realism.
I read a bit about the difference between brown and white fat on wikipedia and what about something like this.
Your creature has a bloodstream. It’s represented just by a tank of water. Different processes put things in the tank and take them out. So you have “blood sugar” and if this is above a certain value your fat cells start to fill up with fat. If the blood sugar level falls below a certain value your fat cells start to drain.
A “secret” fat cell could be a nuclues + x cytoplasm (which in our model makes and breaks fat) + y “fat blobs” (could be vacuoles). The choice you have to make is what is the ratio of x to y. So if you have a lot of cytoplasm and not many fat blobs then you can store and release energy really quickly (so say you come across a massive amount of food which is only available briefly and then none for ages, you want to be able to quickly turn the excess energy into fat) (or say you are a migrating bird, in order to keep flying you need to have a high flow of energy from your fat cells to your muscles and so you need a lot of cytoplasm).
On the other hand if you have more fat blobs then you can store more energy per kilo of fat. So if you went through long periods of grazing followed by long periods of moving slowly (maybe like an elephant or something) then you would want maximum energy density rather than the ability to release a lot in a short burst.
I then thought more about the whole “bloodstream as a tank” thing and why not have the bloodstream as 6 (or n) tanks which are all connected. This is the blood in your body. There is some diffusion between the tanks normally. If you have a heart the tanks get well mixed. You could then connect organs to different tanks (like liver and kidneys which filter the blood, muscles which draw oxygen and blood sugar, lungs which add oxygen, digestive tract which adds blood sugar etc).
For muscle cells there could be a choice between strength (or basically strength density) and ability to tolerate low oxygen. I thought about lactic acid but maybe it just makes sense to have negative oxygen (which represents all chemicals in the bloodstream which will consume oxygen as soon as they can). So you can choose to have really strong muscles that make loads of oxygen debt very fast (so you would be a good sprinter) or you could choose to make weaker muscles which tolerated oxygen debt better and produced less of it (more efficient combustion maybe) and so you would be good at distance running but not so good at sprinting.
So a cheetah might have sprinter muscles + fast fat release. A migrating bird might have weaker muscles + fast fat release. And elephant might have stronger muscles + slow fat release and a sea turtle might have weaker muscles + slow fat release.
And yeah if you gave the system any random cell it would say “this cell has 3 cytoplasm and 2 fat blobs so can store x energy per kilo and get it out at 3/2 times the base rate”. If the cell had other organelles them maybe the storage efficiency (energy in -> energy out) could be reduced. That way any cell could be easily assessed for it’s fitness as a fat cell.
I don’t know if all this is horribly unrealistic though. I think if we could have like 2 dimensions for each cell type then people would really feel super connected to designing their organism at a cellular level.