As for enemies, 4-frame walkcycle, about 2-3 frame attacks, depending on their behaviour, but most importantly, separate left and right sprites for everything with lighting from the left side. Very simplistic but defined animations. I'll post some of mine in a sec.
ShootR(2 frames), ShootL(2 frames), WalkR(2 frames), WalkL(2 frames)

ThrowR(2 frames), ThrowL(2 frames), ChargeR(4 frames), ChargeL(4 frames), walkR(4 frames), walkL(4 frames)

As for backgrounds, simple but defined tiles, preferably using 1 main color per tileset (as in 2 shades plus white, black, light gray, dark gray). I mostly want castle/mantion stuff, so stones, wood but also posh and colorful tiles. Background objects like banners, paintings, windows, candles etc. Platforms could be chandeliers, tables, shelves, stools, whatever. Some slight steampunkish element could work, I mean I already made moving platforms and use a light switching mechanic.
Think modern Castlevania games, but less gothic and more happy.
For wood, neat practice is using brown as light and dark red as dark.
Oh yeah, and as you probably noticed, all the characters have cartoony black outlines, though it's not required for inanimate objects.