The appeal of cube, to me, has always been the pursuit of creating a "perfect" draft environment for play. I'm probably never going to reach that point, but the journey has been interesting enough to keep me tuned in to each release of new cards to tinker around with over the last 6 years. It's just fun being able to play with new toys and plug up any holes that you might have had in previous iterations. I'd say the biggest thing is to really define your goals for an environment and strive to refine that with whatever tools you have available. It doesn't have to be absolutely perfect, and probably won't be for many iterations (if ever), but just keeping your design goals in the back of your head is the best advice I'd give. It's easy to get burnt out or lose passion if you focus on tinkering and refinement way too much, especially if most of your presumed outcomes are theoretical without any real life cube sessions to pull data from. Just go with the flow and keep the process fun. If it isn't, then just take a break.
On that note, I do find that it becomes easier over time to lose sight of initial goals when you first constructed your environment, especially with current design philosophies at WoTC. I wanted a synergistic environment focusing upon design elements rather than raw power, though still at a high power level. For the most part I've kept to this, but there has been a gradual trend towards using the best of each set and even just swapping and moving things around just for the hell of it. There's so much of a push towards hyper efficiency and value that these are often rolled into new legitimately interesting designs and lead to a crappy conundrum. Do I include this card because it's fills in a hole even if it's just generically powerful? Will it be used for the purpose I envisioned or is it just going to be snapped in draft because it's strong? With so few opportunities to actually play in real life (especially now), it's hard to really gauge the effects of any changes made to my environment. Like, it took all of one session with a
Birds of Paradise into
Oko, Thief of Crowns to realize that that idiot was busted and not fun at all to play against. Similar experiences with other high-powered sacred cows in traditional cubes have led to similar conclusions.
I adore designs like
Tireless Tracker that synergize on various fronts but despise extremely pushed bloat like
Questing Beast. I begrudgingly run it due to the niche it occupies as an aggressive, evasive green 4 drop and splash planeswalker hate. I wish it were more on the level of
Vengevine; just a clean and simple template that has more depth beneath the surface once you can exploit synergies with other cards. That's usually my go-to when looking at cards for inclusion. Sometimes it doesn't even have to be something especially compelling or wow-inducing to fill in the needs of an environment. I've slotted
Herald of Torment in and out of my cube multiple times over the years in favor of cooler new aggressive black 3 drops, but it's always been reliable. I need an aggressive 3 with evasion that is good early and has utility at the end of games too, this card has always filled that need. It doesn't always have to be an arms race with the latest hotness coming out in a set. If it doesn't actually improve gameplay or allow you to explore new avenues, maybe it's just not worth messing around with in the first place.