As it has been 8 years since the last summoning mechanic has been introduced (the longest gap between mechanics by 2 years) it seems clear to me that Konami doesn't want to introduce another. Say you were approached to design the next new card type, but there's a catch: Konami want's to keep the learning curve down as much as possible while still offering something that is satisfying to the existing playerbase. What would you design?

Here's one I cooked up as a quick example, don't take the actual card effects too seriously, they're just placeholders to get the gimmick of the mechanic across.
>Split Monsters
These monsters can be either Xyz summoned or Synchro summoned like normal and behave like normal Xyz or Synchro monsters, but if you can attach materials to them after Synchro summoning they gain bonus effects. The idea is to have them be modal cards where in scenario 1 you might want to Synchro them but in scenario 2 you would want to Xyz summon them, letting the extra deck shift back in a more toolboxy direction without having to increase the size of the extra or tack these type of effects onto already powerful in-engine boss monsters as bonus effects.