Another AP Extreme will work. Put it in bridge mode, connect it via Ethernet to the router and you will have AC over a wider area.
If you are using Express in the wireless extend mode, it eats of 50% of the capacity of both the Express and Extreme radios handling uplink traffic. With Ethernet to link the remote to the router, 100% of the radio capacity is available, and could effectively increase the throughput from the Express to Extreme up to the 100Mbps Ethernet speed of the Express. But since Extreme has 1Gbps Ethernet ports, it will be far faster than Express.