These types of power would have to balanced somehow, possibly through fuel costs or the danger of explosives, or both.
Came up with this, in order to least to greatest.
1. xm3.4: what we have now
2. turbine cell: can use anything for fuel
3. fission cell: uses refined ores for fuel
4. RTG cell: uses refined ores for fuel, chance to explode on damage
5. fusion reactor: uses refined gas for fuel
6. SGIF reactor: uses refined gas for fuel, chance to explode on damage.
Names and all that aren't final , the reactors and cells might wind up with the same fuel or whatever. (Actually, this (fuel type, explosive) might wind up being a property of the blockBehavior.xml file)
Cell-type power blocks work in the following way: they can produce up to x energy/instant if your ship energy is about to drop to zero. In other words, they are batteries that must be refueled, not recharged, and with a limited discharge rate. They do not waste energy.
Reactor-type power blocks work in the following way: as long as they are fueled, they produce incredible power, but burn through fuel at a constant rate. In other words, they increase power regeneration by a constant amount for a given period of time.