Tense

1. It will be good if you come to the party.

Grammatically correct, but I would not generate this sentence spontaneously.

2. It would be good if you came to the party.

Grammatically correct. I might generate this sentence.

3a. It would be better if we leave right now.
3b. It would be better if we left right now.

3b. is better. 3a. is possible. I would generate: We'd better leave right now.

4a. We could play poker this evening if you come.
4b. We could play poker this evening if you came.

4a. and 4b. are both possible. I don't find that I have a preference, well, maybe a slight preference for 4b. I would probably say: If you come over tonight, we can play poker. ("over" is "to my place" in this context.) But you didn't want a sentence with "can"! Sorry about that!

Please! No apologies necessary for the example sentences. You were establishing a certain pattern that you needed to get your real questions answered regarding the grammar. I've been through all this myself before when I learned another language, so I understand completely where you're coming from.