Howdy! 
That would be an ET Marathon Mode from (a tentatively named) ET Freestyle. But by giving it in an ‘ARCADE’ type of experience you’d have point multipliers for each successful ET (mission) you finish. You’d have a point multiplier streak (though this should probably be capped after so many ETs, the point is keeping a winning streak). Even if you get an ‘Agent Down’ you’ll still get a final point tally.
Although, I think it’d be something to work towards rather than an option straight away (like a mode unlock).