There are many players that enjoy defensive missions and would rather focus on base-building than rushing against mission timers; players that will tell you "I defend" at the start of Dead of Night map and will enjoy doing that. Ideally, there would be a wide variety of maps, each enjoyable in its own way but playable by everyone.
Some players don't like auto-scrollers, missions on rails that you cannot speed up in any way (e.g., Temple of the Past, Mist Opportunities or Oblivion Express). It might seem that survival maps would suffer from this, however, they don't need to. Players could be given an option to speed up enemy spawns, making the mission faster and upping the difficulty. Miner Evacuation works a bit like that, but it's not a survival map. Alternatively, the part featuring infinite survival could be optional and at the end of mission. Dead of Night could have worked that way. Unfortunately, performance issues and the lack of scaling for attack waves meant it couldn't fulfill that role.
There are many maps, modes or games that could serve as an inspiration for full-fledged survival maps (several campaign missions, Blizzard's own Co-op survival map Left 2 Die; games like They Are Billions, plethora of arcade maps and even horde modes from other genres). The original Last Stand mission in Legacy of the Void campaign was thematically very fitting for infinite survival-type mission. You had to destroy Zenith Stones and lure enemy on Shakuras. Both offense and defense were required. And more importantly, staying alive longer made sense with respect to that mission.
Let's focus on the other variant where the survival is optional and at the end of mission. One option is to have a soft timer for the final objective. With that I mean players have to finish before they get overwhelmed or lose because of other condition over which they have control. Preceding objectives could have standard hard timers – players are required to accomplish objectives before some arbitrary timer runs out. Hard timers might be useful as it's better to let players lose than let them play without a chance of winning. However, Dead of Night gets away without any hard timers; players can only lose by getting overwhelmed and losing their base.
Bonus objectives could be used to put emphasis on the defensive part of the game. One bonus objective could open more enemy spawns and increase pressure on defenses. Additional bonus objective could give experience periodically and encourage staying alive longer.
The downside of this approach is that both players need to stay in-game for the survival part. Another option, which require a special action from each player to win, is less elegant but lets one player to leave (and win), while the other can stay in-game. For example each player has to order a transport ship to lift off, or deliver his own cargo to win.
Few things required from the map or such that would make the
survival aspect better:
- Soft timer on the final objective
- Interesting defensive positions and choke-points; decisions whether to defend natural expansion or not
- Attack waves sizes and frequency increase with the mission time
- If there are any special units attacking, increase their frequency too (e.g., Nydus Worms or Stank)
- Small periodic experience bonus for staying alive longer
- Personal best time shown on the character profile
- If best times were meant to be compared, the main difficulty cannot come from basic Co-op attack waves as they vary in strength too much
To sum up, challenging (infinite) survival mission would be a good addition to the Co-op map pool. And it doesn't have to be an autoscroller or interfere with the standard playthrough of the mission.
