Now this is all a matter of opinion, but here is my 2 cents.
1, What makes an SCP Game scary?.
A number of things, these include, but are not limited to.
Atmosphere, just throwing dead corpses and jumpscares everywhere detract's from the scariness, in fact, the only game i can think of that does constant jumpscares well is SCP-087-B, throw in a few ambient sounds and some dark creepy moments and your game will be fine, as i think of it, its easy to jumpscare someone, its harder to actually disturb them.
The SCPS, one does not just throw all 2000 (Or 1999) SCP's into one game, obviously, your choice of SCP's will effect the games atmosphere and scariness, where SCP:CB has a ton of SCP's it uses most (Not all) in different ways, how about a chase scene with 682?, or something similar, just make your choices make sense, SCP's like 087 are not contained in any site, they are inanimate and cannot be taken from the place they were discovered in.
The sounds, all scary games need some scary ambience, like SCP:CB, which does this almost perfect (In my opinion),
Have creepy rumbling, and other noises, and have each SCP have there own ambient sounds.
Location, Even in a containment breach, the SCP's that escaped must have a cell somewhere, like SCP-173's containment cell, have them similar to there files containment procedure so they make sense, unlike SCP-895 in SCP:CB, which clearly breaks containment protocol.
2, What should the base elements of the game be?.
I completely agree with Sneakyferret, its completely obvious an unarmed human, (Or even armed for that matter) wouldn't stand a chance against most SCP's.
3. What sort of location do you think it is suitable for an SCP Game to take place in?
It depends, do you want to make an SCP game from SCP's like 087 or 432?, they are pretty much locations themselves, or have it be a containment breach, are you the MTF entering to contain the escaped SCP's?, a doctor who survived the breach and must escape?, or a simple class-D, or maybe something entirely different.
4. What SCPs should be included that aren't basically useless?
This is something i can't answer, like i said earlier in my post, just make your SCP's make sense, and your pretty much good, have them be used for something innovative, or have clever puzzle to solve, if you implement them right, no SCP will be useless.
Anyways, sorry for the long post, this is only my opinion, i don't make SCP games and have no skill in programming of any kind, i just thought i'd share how i feel an SCP game should be made.