The replayability question feels important for a word game. Are you planning different round modifiers or changing objectives so repeat runs feel meaningfully different instead of just faster or harder?
Yes, replayability is one of the things I'm looking at now.
I want to keep the core loop simple, satisying and endless rather than add lots of changing objectives. Block Blast is one reference for that rhythm: mostly stable rules, with occasional situations that force you to think more carefully.
For Lex Blast, I'm experimenting with periodic challenge rounds, for example starting from a single central anchor and a more constrained rack, to create those moments of extra tension without changing the core game.
Do not hesitate if you have ideas or recommendations.
The periodic challenge-round idea sounds like a good fit if you want to keep the core loop simple. One thing you could test is a small rotating rule modifier, like limited rack space or a temporary scoring bonus for a certain letter pattern, so the challenge changes without changing the main game too much.