Take a note out of mech touch and summoning magic from fantasy. You put them together like jojo's stands or like power rangers but fantasy-like. You summon a mech/giant robot to fight for you.
Have start off with novices with weak "modern day" robots then move on to crazier and crazier bots.
A possible sub plot of the story is "are the bots just machines or something more?"
Then take the knights and magic approach and constantly update and improve the protagonist's bot. You might use a system similar to the mech touch as well. Maybe keep all the knowledge in tech trees.