Depending on zoning and lot size it's likely a new home would be built on the adjoining lot by a developer or buyers looking to build a custom home. If the lot is smaller than a standard lot or irregular in shape it may not be as attractive to build on and could remain vacant. Assuming it's a standard lot and is zoned for a single family home, you can expect a home to be built there. If the lot is priced too high it may not sell for some time or the owner may take it off the market and sell at a later date. Alternatively it could be bought by an investor to sit on. 10 years is a long time to hope for nothing to happen.
A lease with an option to purchase may work but it would all depend on the terms offered. The seller may want to work out a deal on the vacnt lot if it means he can sell the home to you (assuming both properties are owned by the same person). Otherwise, expect construction at some point during your duration of ownership should you purchase.
Best of Luck, Ken.