I always have a splash card, usually a 3* or 2* card. Chiara cr is a good example, fits my style of play.
Each week is also different, with new bans and other clans maybe having more of an advantage than the others it's good to see which are popular in that certain week and work your way around it. For example, Roots could be trending in a particular week and me personally, I'd probably then run with a Skeelz deck.
High power or power manip cards are good, that being said I'm not so keen on Bangers.
And what the Ginger guy said too
A splash card is a card that can be placed into any deck, regardless of clan usually. Say if you was playing a monodeck and 7 of those cards were Montana, your splash card could perhaps be Lehane. It's a 'splash' card because it just fits well, if it's ability was support or was a card largely reliant on a clan bonus, it wouldn't be a splash card. Other popular examples are Chiara cr, Caelus cr, Spiaghi etc...
Main things to consider when building an ELO deck
1) What is your playstyle? This is key because some people play slowly and carefully, while others play hard then recover
2) What types of cards/clans work well for you? If you are really good at predicting others' plays, pill manipulation might work well for you, for example
3) What is the meta? You can either go with or against the grain. If everyone is using Rescue, use Rescue and outplay them or use Piranas/Nightmare/Raptors and counter them
4) What type of deck works for you? Some people play an even split, some play mono, some splash with cards like Spiaghi, some splash with 2-3 cards like Chiara Cr, Redra, Beck - Nena, Plunk
Look for common bans that aren't banned this week and work from there if you want to actually put a effort in actually getting to the top.
Look at popular decks from the clan and either copy or build based on preference.
I usually make what I like so I wouldn't really know. But I would prob do the following if I wanted to tryhard.