http://www.nexusmods.com/xcom/mods/79/?This lets you skip the revealed alien free bullshit move thing and the reveal cut-scenes but I preferred it a step further where basically all aliens start spawned on the map and were already on high alert/aggro mode, I think its called patrol by the game. If you are just taking away the free move and not giving them any help make sure you have cranked up the difficulty and optional difficulty settings for sure. Its also just a great mod that gives you a ton of features you might need/want even if you use some of the big game overhauls and its a really clean interface for turning settings on/off and stuff. I loved it and always used it no matter what else I had going. But I also like to RP in my head and therefore its really useful to be able to control things like what countries my next batch of recruits comes from and whatnot on the fly. I always hated it when China would be super pissed and about to withdraw and then RNG gives me 4 chinese dudes. Some kind of OCD trigger.
In order to have more aggressive AI there are a ton of choices. Basically ever major mod aims to fix the glaring Lightning Reflexes + Squad Sight + Overwatch and never advance positions with more than one marine a turn ever doctrine. So anything claiming to make the game harder or more complex will do that. I really enjoyed Long War but it was 1 of 3 major overhauls that existed back then not sure if more have been made since EW came out.
Here's the smallest ones that I saw on Nexus just searching for the word patrol (so for sure I missed some, "spawn" might be another good search string or "aggro").
http://www.nexusmods.com/xcom/mods/470/?This could be pretty crazy. He's randomized every single spawn that he can I think and put them all on patrol. Which means they aren't just sitting there in a spot waiting for you to discover them. So sounds like a shitton more RNG. Which I like but other people might hate.
http://www.nexusmods.com/xcom/mods/13/?This guy tries to make Snipers not the one true god of xcom and fucks with grenades, I'm not sure I'd like making alien grenades stronger because they are already pretty beastmode when your used to fighting several spawns at once. But he hasn't done too much and it looks like he's trying and rebalancing with several update tweaks. Might be good.
http://www.nexusmods.com/xcom/mods/102/?This one is meant to make things a little easier while unshackling you from being terrified of spotting aliens because then they get the stupid magical switch to full combat mode and the free hide move. While keeping the hide move mechanic, which I have never tried. Could be interesting since you want to stay really close to vanilla, he's the only guy who isn't tweaking a single other thing.