My last campaign was started in 4 to 6 month arcs, between which we usually modify the system and rebuild the characters by using a custom Work-In-Progress system. My players and I have been curious to use D & D a few times, but the upgrade curve bothers me.
My problem is that I do not expect players to gain more than 3 levels during an arc. Most classes earn only one major feature per level that may not be of interest to the player. Most digital features increase every 2 or 4 levels. Ditto for spellcasters unlocking a new spell level. This is particularly problematic because in our current system, the upper levels offer many options and very useful features. Closer to the effect of a new archetype or an exploit.
For this reason, I would like to allow players to choose 2 levels of character instead of 1 at a higher level. To ensure that their functionality improves and that they get at least one news that interests them. If they like everything, so much the better. During a circular arc, instead of going from level 3 to level 6 (getting a feat, improving damage to 5 and usually working without a fight before the final), they went from level 3 to level 9, thus allowing to unlock the generally much more powerful characteristics of level 9 and / or 10.
Are there any unintended negative consequences to this?
For the record, we use Milestone leveling for simplicity. Players can expect at most three major levels during this period, as they have generally given new abilities or huge power spikes. The game itself is usually lightweight in combat, although a game of D & D would probably have more.