If you do plan to go for any Fusion, spend your money on energy packs, rather than on shard packs.
Start collecting as many Rares and Epics as you can get your hands on. You never know when you will need them for a Fusion. Having an Epic (usually quite useless one) already will save you a LOT of energy. If you haven't fully upgraded your Gem Mine, do so as soon as possible. It will pay back. You will also need a lot of Gems or a huge amount of silver to upgrade your champion roster and Vault capacity.
If an Epic required for a Fusion is a really good one and you already have him/her, consider fusing another one, resources permitting. Basher and Fayne come into mind from the Fusions I have done. I actually had to stop at Fayne, because I lacked some Rares to complete the Legendary fusion she was a part of.
Get your three Tag Arena teams together as soon as possble, no matter how strong or not. Try to obtain at least some gold bars every day. Even in the lowest tier, you will be able to eventually buy some nice stuff with the gold bars, such as Energy and Ancient shards (with a very long cooldown). You will also be able to start collecting fragments for Drexthar Bloodtwin, who is a great Legendary, one of my most favourites for most of the dungeons.