As alternatives to vertical retro-engines landing:
horizontal landing using wings, already discussed in Why is it preferable for SpaceX to land their booster vertically rather than fly it down with wings?.
horizontal landing using using airbags, not yet discussed.
TSTO, the India project for reusable launcher, plans to test both vertical and horizontal options.
SpaceX is trying to shorten the distance between the landing site and the launch site for shorter launch cycles, so the use of the controlled landing, but it seems also possible to land horizontally without losing this advantage.
To prevent a crash when reaching the ground, the sea or the barge, why not just switch, 30s before the landing, from thrust-controlled trajectory to an unpowered horizontal landing with parachutes and airbags)