Autocomplete
Allow suggestions to speed up the query, not take over.
Good autocomplete helps formulate, corrects where necessary and provides direct routes to relevant products and categories.
Consciously mix suggestion types
Visually distinguish between query suggestions, products, brands, and categories.
Limit noise
A short relevant list works better than dozens of almost equal suggestions.
Support keyboard
Arrow keys, Enter and Escape must work predictably.
Protect input
An automatic correction should not tacitly change brand or SKU input.
With selection
Track which suggestions are chosen and which types are ignored.