Hard to tell what you have, my guesses include alder, witch hazel, american hazel, hornbeam, and Elaeagnus species of some sort. Possibly Rhamnus species - buckthorn. Need leaf pictures to go with bud pictures.
Russian Olive - make good bonsai - small, sweet smelling white flowers, blue-ish foliage, fast growing, and extremely cold hardy. Really rough bark on older trunks.
In fact most Elaeagnus make reasonable to good, sometimes great bonsai. By great I mean a few have been exhibited in top ranked bonsai shows in Japan. There it is usually Elaeagnus pungens or E. multiflora
Elaeagnus angustifolia - Russian olive - leaves are narrow, like a willow leaf. Very rough bark on wood older than 10 years.
E. umbellata - Autumn berry - fruit ripens September and hangs into October. Relatively smooth bark even on older wood.
E. multiflora - Autumn berry, or silverberry, - this one in Michigan fruit ripens in August - smooth bark, leaves are blue-ish, wider leaf than russian olive
E. pungens - Silverberry or Goumi berry - this one is only hardy through zone 7, maybe into 6b. Has the largest fruit. Grown as a specialty crop, a novel ''superfood''. Flowers in autumn, fruit ripens late spring or early summer after hanging all winter.
