In Space Quest IV you get to buy (actually it's required to beat the game) a hint book for Space Quest IV. However, most of the hints are useless, and why wouldn't they be? Thanks to time travel, you actually spend most of the game in Space Quests XII, X and I!
Also in Space Quest IV, the Sequel Police will pursue Roger to every time period in the game—except for Space Quest I. Why? Because Space Quest I isn't a sequel.
For a comedy series, Space Quest had a particularly vile one. In Space Quest 4, Roger is tossed through time by his son, a battle-scarred fighter for La Résistance at the age of 19. Roger's home world of Xenon is a post-apocalyptic nightmare full of Sequel Police terminators, killer robots, and any living being found given a lobotomy and having their eyes wired open, run by a supercomputer corrupted by Roger's Archenemy. After Roger dispatches the Vohaul computer, his son says they had to go back in time in order to find "the only man who defeated Vohaul." That means Roger is stuck in a Stable Time Loop - he remembers what happened (as we see in Space Quest V), but he can't prevent the horrors he saw from happening...and Junior heavily implied that Roger and Beatrice both don't live to see him grow up.
Another bit of Fridge Horror is in The Next Mutation where you realize that the Corrupt Corporate Executive has been regularly bribing Quirk to dump the mutagenic sludge. We saw the devastation it wreaked on Klorox II, but how many other planets did this stuff get dropped on? Even better is that a ship the size of the Goliath should have dozens or hundreds of crew members, but only a dozen or so are rescued...and there are puddles of sludge everywhere on the ship, indicating those are the crew's remains. Seriously, Two Guys, this is supposed to be a comedy?
A lesser one, but still unnerving. just how long was Roger in that stasis unit between Space Quest 2 and 3? It was long enough for a chintzy gewgaw to accumulate 400,000 buckazioids interest from Gippaziod Novelty (though it may be more due to Loan Shark tendencies). For that matter, the same company who keeps hassling Roger over the interest is the same one who put together that deadly slot machine in Ulence Flats. Do they usually kill their customers, and Roger's just been 'lucky?'.
Kind of an obvious one, but SQ6 ends with Roger seemingly deciding to leave Beatrice for Stellar. That means that unless Roger Jr. has already been conceived before the events of this game, he'll never exist. That, or Roger will cheat on Beatrice while remaining married to her. Jerkass move, considering he already knows he's fated to have a son with Beatrice.
Roger may be hinted as being philandering as early as Space Quest IV (or possibly as late as Space Quest X) with the Latex Babes of Estros. Also, there's nothing set in stone, since one of the game over screens in SQV is Roger becoming Ret Gone.
The crush is more one-way on Stellar's part, and Roger does let her down as gently as he's able. It might end up as a case where Roger and Stellar do love one another, but not romantically. Furthermore, the Latex Babes were Yandere types who wouldn't be able to take a "no" answer gracefully.
Let's go down the list. Stellar has a pill stuck in her larynx, an unchewed Twinkie stuck halfway down her esophagus, a stomach ulcer so bad it corroded a hole through her stomach lining, three unchewed M&Ms in her stomach, an undigested celery string stuck in her stomach lining, a staple perforating it from the inside, a gallstone, a tapeworm in her small intestine, and in her appendix: a paperclip, a silver tooth filling and a penny. Not to mention the tiny robots wreaking havoc on her stomach and digging into her brain. She should be dead.
In the Space Quest 1 remake, you can crash into another Sierra game (Conquest of the Longbow IIRC), and there's an offhand comment in your death screen that if you were in your own universe, the life-threatening injuries you sustained would be easily fixable. Maybe the same principles apply here.