If i remember well, the devs worked on something very similar but weren't able to finish it for the release. I hope we will see it soon.

well I wouldnt use Left 4 Dead to descrive Enfos as they are quite different to describe!. 

Enfos, you have to stop mobs from reaching your center (goalline) point with just your hero (no towers). The other team does the same, so in essence its survival mode (like L4D). But you cant actually directly attack the other players heroes (aside from some abilities) and the two team can not physically reach the each other. So its strictly player vs mobs. What the other team can do to increase there chances, is to use mana to summon temporary buffs for the mobs pouring out of his rival's portal or stun you for a real short time.

The goal would be to outlast your opportents or just see how far you can go by yourselves. So its basicly demigod but without flags, and enemy demigods facing off.  This makes the experiance much more relaxing since ganking is pretty hard to do, especially at the start when the weak mobs are pouring out.

I honestly can't see many DG's being used besides TB (now some of those "let's slap on all the AoE moves despite the fact that RoI IV kills everything but super buffed minions" might be useful), and maybe rook (tower farm to slow down creeps) and UB (Ooze + high health regeneration = weaker mobs for him and everybody else). Not many other DG's have good AoE options; Sedna seems totally worthless, Erebus doesn't have great AoE though with an "infinite mist" build you could theoretically get one DG to infinitely kite the minions through the mist, Reg's AA is probably enough but mines are also good, etc.

Well yeah, the DG's will need to have there skills redone (like why use silence?). But I still think that it could be created, not this year but mabye towards the end of next year or following years.

They would need to create new:

-hero skills (ex. silence useless, TB circle of fire op)

-map (you only need a single map, though additional maps with variations on the amount of lanes would be nice)

-new color schemes for mobs

I think that those would be the base changes requried to get it started. (of course I only took like a few minutes to think it over, so I probably missed a ton like additional sounds, etc.)

Anyway this would be great for an expansion (due to the work involved). Perhaps a year or 2 down the road, an expansion that incorporates a lot of mods that were pretty popular in WC3 would be released using the great Demigod engine. Tower defense and tower wars were fun and very popular, and I feel this engine could be used to great succes.

Its just I feel its a shame that Demigod while being well made and fun, is very narrow in what it offers.

I think we'll see silence become a more viable skill as 5v5's begin to take place more often. Having all surrounding enemies unable to cast for 5 seconds is a bigger deal than 1300 healing in a 5v5, imho.