Curious, I went to the Grand Comic Book Database and looked up the villains in the Bronze Age Action Comics. Very few new costumed supervillains or recurring adversaries of any stripe in evidence. The best new characters of the Bronze Age are the WGBS supporting characters, and new super-friends like Captain Strong (really, an extended joke), Vartox and Superwoman. There are a few super-mischief-makers as well.
The best Bronze Age-specific villain remains Terra-Man, in my opinion.
Here's the Action list. It is incomplete --someone really needs to finish indexing that series --many characters and plot synopsis are missing.
Action395-dec1970 Althera
396-jan71 the Seekers
400-may71 Gregor Nagy ("the Changeling")
-Brainiac; Yllura; Arvor
404 -sep71 caesar
406-no71 Van-Tarr
409 -feb72 Shalox
411 apr72 -Titus J. Person
412 may72-Josdin
413 jun72 -Dr. Mystir (really Brainiac)
415 aug72-The Metropolis Monster
417 oct72 -Brainiac; Lex Luthor; the Marauder; Grax (Crime-Lords)
418 nov72 -Brainiac and Lex Luthor. Also: the Marauder
420 jan73-Towbee
424 jun73 -grodd
425 jul73 -super-moa
426 aug73 -terra-man
428 oct73 -luthor
430 dec73-chameleon creature
431 jan74-chameleon creature
432 feb74 -toyman, toyman II
433 mar74 -Noam Smythe
437 jul74 valdemar & effron
439 sep74 -carnox
441 nov74 -the Weather Wizard
443 jan75 -queen bee (zazalla) Brainiac (villain); Clayface (villain); The Harpy (villain); Merlyn (villain); Ocean Master (villain); Sinestro (villain); Chronos (villain); Gorilla Grodd (villain);
444 feb75-Sinestro (villain); Zoyoz (villain); Kydyk (villain);
445 mar75 the Superman Revenge Squad
454 dec75 toymanII
455 jan76 -junkman
456 feb76 the shark
457 mar76 -Whirlicane
458 apr76 -blackrock
459 may76 -blackrock
460 -jun76 -mxy, Karb-Brak, an alien from Andromeda
462 aug76 -costumed supervillain?

463 sep76 -ditto
464 oct76 -pile driver
465 nov76 -luthor
466 dec76 luthor
468 feb77 terra-man
469 mar77 terra-man
470 apr77 terra-man
471 may77 -phantom zone villains
472 jun77 -pzv
473 jul77 -pzv
474 aug77 dr light
475 sep77 Vs. Vartox, 1st; with Karb-brak
476 oct77 vartox
477 nov77 land-lords of earth
479 jan78 giant from the golden atom
480-83 feb78-may79 amazo
484 jun78 earth2 wizard
487 sep78 microwave man
488 oct78 microwave man
489 jan79 brainiac
495 may79 ultimate warrior
502 dec79 galactic golem
503 jan80 The Astounding Kolzer
504 feb80 mystery marauder
505 mar80 jorlan
506 apr80 joplan; orn-zu
510 aug80 luthor
511 sep80 luuthor. terra-man &

512 oct80 luthor
513 nov80 luthor
514 dec80 luthor brainiac
515 jan81 vandal savage, luthor
516 feb81 vandal savage
517 mar81 nemrul
519 may81 "cosmic hunter"?
521 jul81 vixen & mule
522 aug81 clockwork man
523 sep81 aliens
525 nov81 neutron
526 dec81 neutron, HIVE
527 jan82 Lord Satanis (intro, villain); Syrene (intro, villain)
528 feb82 -530 apr82 brainiac
531 may82 Frankenstein's Monster
532 june82-jul82 533 HIVE
534 aug82 Lord Satanis; Syrene
535-536 sep-oct82 the mole
537-538 nov-dec82 jackhammer
539-541 jan-mar83 Syrene (villain); Lord Satanis (villain)
542 apr83 vandal savage
543 may83 neutron vandal savage
544 jun83 new Luthor and Brainiac
545-6 jul-aug83 brainiac
547 sept83 planeteer
548 oct83 pz villains
549 nov83 zod squad
555 may84 parasite
557 jul84 terra-man
559 sep84 yellow peri
561 nov84 toyman
562 dec84 Queen Bee (villain); King Alexander (villain)
563 jan85 mxy
564 feb85 master jailer
566 apr85 "sea hag"
567 may85 yellow peri
573 nov85 j wilbur wolfingham
575 jan86 intellex
577 mar86 caitiff the vampire
578 apr86 vandal savage
583 mxy, legion of supervillains, brainiac, luthor, kryptonite man