CSB Nextmap Info keeps the upcoming map visible. It reads amx_nextmap live (so it reflects a map vote result), shows it in a small HUD line, answers /nextmap in chat with the time left, and announces the next map at fixed thresholds — 5, 2, 1 minute and 30 seconds — so nobody is caught out by the change.
amx_nextmap./nextmap chat command with time left.csb_nextmap_hud.sma, copy it to addons/amxmodx/plugins/ and add it to plugins.ini.amx_nextmap (the stock nextmap.amxx does).