You can buy/earn a base card of a player and power them up through the tiers with another currency like training in MUT or do stat grinds online/offline.
Training, or whatever you want to call it, could be acquired through exchanges or getting training packs as rewards in RS. Training could also be in packs.
For exchanges, the overalls of players would determine the training value, this could make the market more interesting.
Power up program is huge in MUT and adds a ton of variety in building lineups. This game has the potential to take lineups to the next level.