It's so easy to get caught up in all the bits and pieces of everyday life, and sometimes things that at one time seemed important were shoved aside. Then you remember, "Gee I really did want to learn to play in 19/8", or something of that nature.
So let's take 19/8 as an example. Is the missing skill because of no opportunities to play in that time signature? Is it a lack of understanding? Perhaps you don't feel it internally, naturally, and so are hesitant to give it a try.
Here's a simple and painfully obvious solution: Just do it. Take that thing you wanted to work on but never got around to and spend just 5 minutes working on it. Now, that amount of practice won't make you gig-worthy, but you'll be a lot closer than you'd be if you hadn't spent those few minutes. No amount of thinking or wishing will get the job done.
I find that I can get a handle on a lot of things just by doing them, and by that I mean sitting down at the set or pad and doing a mini workout. So I might spend 5 minutes playing a very straight 19/8 pattern. Or 5 minutes of a particular tom pattern. Doesn't matter what it is. Nor do I have to commit a half hour or an hour every day for the next week or month. The mere act of doing makes a difference.
I also find this approach less stressful than trying to master something. It's possible to ‘over learn’ things and that can actually work against you.
So next time you find yourself wishing you could play something you’ve not gotten around to, stop wishing and start doing.