The trouble is, the firther investigation has to be linked with the ACTUAL investigation you have done, not just with the same topic (i.e. osmosis). You do not say exactly WHAT variable to do with osmosis that you have investigated, or have you just investigated 'factors' that affect osmosis?
If you have just investigated factors, then you could describe an experiment to investigate a SECOND factor, e.g. if you have investigated surface area, you could investigate substrate concentration. Your description of the experiment would have to be detailed, though, e.g. apparatus, method (including control of variables).
If your investigation is (e.g.) specifically on surface area, then you would have to do a further investigation which is specifically to do with surface area. For example, if you have 'measured' surface area by just how many discs a potato cylinder is cut up into, you could measure tha actual surface area to try to establish a mathematical relationship between surface area and rate of reaction.
If this does not help, you will need to send in more details of exactly what you investigated and how, so I can give more specific help.