Paranoia.

Reminds me of Romancing SaGa, where certain events would happen, sometimes closing off sidequests, based off how many battles you fought. (Note: That's an oversimplification; the strength of the enemies you fight also matters.) At least there you would never be unable to complete the game, as the endgame quests never expire (though you do need to fight enough battles to get the event rank high before even starting them), but it is still annoying and discourages fighting battles in a game where combat is fun.

From what I hear, Avernum 3 has events that happen at a certain in-game time; if you take too long, eventually a certain place comes under attack, and unless you save it in time, you will get a game over. Also, the world's condition worsens if you don't do certain main quests in time (though they never become impossible).

If one really wants to use such a mechanic, I think basing off completed quests would be a better approach, and making it so that running out of time to complete the game is not a possibility. (In Romancing SaGa, you can run out of time to complete certain sidequests, but you can't run out of time to beat the main game.)

System mechanics, I've always been a fan of BRP. World wise, Nightbane and Vampire the Masquerade. DnD 3.5 isn't bad, though after about level 8, the combat starts bogging the game too much.

I like AD&D 2nd Ed. Don'tr care for latter editions of that game. Too snowflakey for my tastes. To much placed on
being a super hero by default. In the 2nd ed you had to earn your abilities by being smart. Not by just showing up.

My favourite system is 1st Edition Palladium Fantasy rpg. Combat is simple and I think it reflects actual combat conditions. Limited initiative rules. WFRP 1ed had some pretty good initiative rules but overall I didn't like it.

One major niche system is Dragon Warriors by Dave Morris. Its a bit like D&D in many ways but simplified and more fluid. The Author created a nice fantasy world with parallels with actual 12-14th century, reimagined world with fantasy creatures and magic.

I own AD&D 1ed hard copy plus the drivethru soft copies but I think its awkward if I had to run a game. So many rules for everything in small print. I respect it was an original gygax system that was in place for more than 10 years?

Gee, why don't we do a forum game on gog? Don't think the mods would mind.. Too much work mind you, I often ask the GM to make rolls for me.

In 3.x, at least, to really make an optimized character build, you have to be smart. There are a huge number of possible builds in those games, and some of them (multiclassing spellcasters without prestige classes, for example) just aren't that good. On the other hand, a smart player can find ways to break the game.\

Let me remind you; a build like Pun-pun is something that a stupid player is not likely to figure out. (For anyone unfamiliar with this build (which no sane DM should ever allow), the trick is to acquire an ability called Manipulate Form (which was never intended for player use) by use of a polymorph effect, and then to break the game with it. It is so powerful that it's basically the table top equivalent of Arbitrary Code Execution exploits in video games; the player gets powers that should only be available to the DM, like getting access to arbitrary abilities.)

(As a side note, after a certain point it becomes more fun to come up with these gamebreakingly overpowered character builds than to actually play with them, assuming the DM would allow their use (and no sane DM who's paying attention should allow the Pun-pun build to get access to Manipulate Form.)

BECMI D&D and D&D 5
The Dark Eye 3 and 4.1
Warhammer 40.000 RPG Dark Heresy
Shadowrun 5