I have 3 players for my RTL campaign. Instead of having one play two heroes, I have proposed following rule tweaks:

1. Number of monsters for level/encounter is decreased by 25% (OL's choice, number of removed monsters is rounded down if fraction is 0.5 or lower)

2. Threat cost of cards and abilities (everything that costs threat, including flipping reinforcement marker) is increased by 25% (rounded up if fraction is 0.5 or higher)

3. Players can choose or create their heroes instead of drawing as per rules.

What do you think - is it fair?

Speaking without having actually played RtL...

Reducing the number of monsters in proportion to the number of heroes will likely allow the heroes to proceed at roughly the same pace, but it means that the OL won't get as many kills (you'll only be able to make 3/4 as many attacks, so you'll kill 3 heroes where you would've killed 4 in a normal game), which means you won't get as much conquest (while the heroes will get about the same), and that you'll be less likely to kill any specific hero at a specific time (if it's ever really important that you kill hero X right now to stop the heroes from doing such-and-such). That's probably a problem.

Similarly, increasing the threat cost of trap cards means you won't do as much damage with traps, which means fewer kills and therefore less conquest.

Ideally, I think you want the monsters as a collective to do about the same amount of damage, but for the heroes to dispatch them as quickly as if there were 4 heroes. The obvious way to do that is to have the same number of monsters, but make them easier to kill...but that only works if they normally take several hits to kill. If the heroes can already one-shot many monsters (as is the case in basic Descent), there's no amount of health or armor you can take away that makes them easy enough to kill with fewer attacks.

Alternately, you could try to have fewer monsters but give them stronger attacks. The problem there is that you can't just give them 1/3 extra damage or something, because concentrating the same amount of damage into fewer attacks will make armor less effective (among various other side-effects). So calibrating the damage to an appropriate value would probably require a lot of playtesting.

This is one of the reasons I'm completely rebalancing basic Descent...

On other matters, if the heroes are getting the same amount of treasure as in a larger game, that also means that the average time it takes to upgrade all heroes in the party to a given level of equipment will be shorter, and they'll have more options to pick between (which means the right equipment for the job, or just flat-out better equipment in some cases).

Also, the official build-your-own-hero rules are not terribly well balanced. They probably work OK as general guidelines, but malicious designers can still make heroes that are dramatically powerful or exceptionally weak--especially if the entire party is coordinating their designs, and get to choose specific skills rather than getting random draws.