A multi-tool should, like the saddlebag itself, be compact but have the essential tools to carry out key roadside repairs. In reality this is only likely to be a selection of Allen keys (and, for most bikes, only a 4mm and 5mm Allen key are required) but a chain splitter is a sensible addition.
The Birzman E-Version 15 Function Multi-Tool is a good example and covers most bases. It includes 2.5/3/4/5/6/8 Allen keys,T15 and T25 Torx keys, a chain splitter, 3.2/4.3/4.4/Mavic spoke keys and a crosshead screwdriver in a compact package. It’s flat profile ensures it takes up very little room in a saddlebag, while it’s light (127g), looks great and is well priced at £17.99.