My recent campaign has been run in arcs of 4 to 6 month, between which we usually change system and rebuild the characters due to using a Work-In-Progress custom system. My players and I have been curious about using D&D a few times, but the leveling curve is bugging me.
My problem is that I do not expect the players to gain more than 3 levels during the course of an arc. Most classes gain a single major feature per level that may not be interesting for the player. Most numerical features increase every 2 or 4 levels. Same for spell-casters unlocking a new spell level. This is especially a problem because in our current system, level-ups give a lot of options and possibly very useful features, closer to the effect of a new archetype or a feat.
For this reason, I would like to let players take 2 character levels instead of 1 during a level-up, so as to make sure that their features upgrade and that they get at least a new one that interests them. If they like all of it, all the better. Over the course of an arc, instead of going from level 3 to level 6 (gaining a feat, a damage upgrade at 5 and a usually non-combat feature before the finale), they would go from level 3 to level 9, unlocking the usually much more impactful features of level 9 and/or 10.
Are there any unintended negative consequences to this?
For the record, we use Milestone leveling for simplicity. The players can expect at most three major level-ups during this time, those usually gave to completely new abilities or huge power spikes. The gameplay itself is usually light in combat, although a D&D game would probably have more.
Best Answer
You have to be wary of the spikes in character Complexity
I'm going to use the Rogue as an example because this class represents the most obvious manifestations of how this kind of rapid leveling could be an issue; but in general, this is an issue that affects most classes and most levels of play.
Consider the first level. A level 1 rogue has the following features they need to contend with:
Those are quite a few features, but it's balanced out by the fact that they'll be spending some time at level 1, only dealing with those features. They'll have time to work out how each feature works and how to best make use of them.
When they hit level 2, they're only gaining one additional ability, but it's a pretty significant ability:
Then, at Level 3, comes the archetype features. Every single archetype gains at least 2 new features. Even for archetypes like the Thief, where one of their features just improves upon the Cunning Action feature, the other feature is also giving some movement bonuses (or removing movement penalties).
If they're an Arcane Trickster, then they're getting
Now let's see what this feature gain looks like if we include level 2:
Do you see the problem? There's a lot that a character is gaining in terms of features just going from levels 1 to 3. As a consequence, it's quite easy for one or more of those features to fall through the cracks and forgotten.
Now let's just take this same character and jump them to level 9, without doing the step-by-step, and just see how many new features are showing up:
This is a lot to keep track of! Each of these features have significant impacts on how this character would play on a moment-to-moment basis, and if these are things that the player can't keep track of, it'll dramatically reduce their power level.
This is less of an issue in normal play because you normally advance through these levels over the course of months or even over a year or two. There's a lot of time to work out how each feature improves upon the character they're playing. But compressed into the weekend or two between sessions, it could be too much.
Experienced Players can probably handle these changes—Probably
It should go without saying that if you trust that your players are either experienced enough or smart enough to be able to take these kinds of complexity gains in stride without struggling to keep track of their new features, then you have a lot less to worry about. I know there are lots of players—especially people who are used to playing Rogues—who would scoff at the list I've posted above and say "what, that's all I need to deal with?"
So I'm not here to tell you this is a bad idea, or that your campaign is going to suffer if you choose to do this. All I'm going to tell you is that if you plan to do this, it is very important that your players understand what you're signing them up for, and are prepared to deal with it. If they are, then I imagine this is a good way to quickly get players to a Tier of play that tends to be somewhat underrepresented in 5th edition.
Just be mindful that even experienced players might be playing a class they haven't touched before, and that they could get overwhelmed by having too many new features to keep track of.