It's also possible to use a high gain directional antenna. Either a commercial one or DIY (there are many designs available online). People have used very high gain antennas to extend range of consumer routers to ~1 mile (across a pond).
One very important thing to know when choosing an antenna is what the gain actually means -
Antennas are passive devices, so by the law of conservation of energy, they cannot increase total power output from the router.
All antennas can do is to shape the radiation pattern, to radiate more energy in some directions than others. The gain of an antenna is the ratio of power in the strongest direction to power that would be radiated by a hypothetical antenna that radiates equally in all directions (no real world antennas behave like that, so it's purely hypothetical).
A high gain antenna just means it's more directional (narrower beam). Higher gain is not always better - it depends on the coverage required. Very high gain antennas require very precise aiming, because the beam is so narrow.
100 ft doesn't sound very far, though, assuming there are no metal walls between them. May be easier to just buy a higher powered router if you just happen to have a weak router.
Powerline ethernet only works when both sides are on the same circuit. If it has its own fusebox it most likely won't work.