'Expect' followed by S + will + V?

Looking up 'expect' will bring up discussions of whether it should be followed by the infinitive or a present participle, and the infinitive is the winner.

But what about when using a modal verb, like 'will'

(1) "I expect he will be at the party."

This doesn't sound so wrong. Most examples of 'expect' put it with an object after, followed by an infinitive:

(2) "I expect him to be at the party."

But we're saying something quite different in (2), methinks - almost as though you have either inside knowledge about his intentions to go the party (the expectation of it), or even with a power/hierarchy distance, like a teacher expecting a student (because they were told to, and if they didn't there would be repercussions) to be at the party.

(1) just seems to be a guess, so 'expect' could easily be replaced with 'think':

(3) "I think he will be at the party."

Still, (1) sounds more certain than (3), but not as certain as (2).

Isn't (1) still OK, even if it doesn't follow the 'standard' usage of expect being followed by an infinitive?

I expect the unexpected.

This example contains neither infinitive nor present participle. There is nothing substandard or even unusual about this sentence. It follows a common pattern: subject / verb / direct object.

The same pattern exists in your first example:

I expect [that] he will be at the party.

Certainly, not every verb that licenses a direct object also licenses a complete clause to take that role. The sentence "I want [that] he will be at the party" does not work well, even though "I want the unexpected" is perfectly natural.

Additionally, the verb to expect can license both a direct object and an object complement.

I expect him to be at the party.

We've seen that the verb to want does license a direct object but doesn't license every possible kind of direct object. With to expect, although it does license an object complement, it does not license every possible kind of object complement.

* I expect him being at the party.

In fact, it doesn't even license every possible kind of direct object in isolation. Gerunds fail.

* I expect his being at the party.

Strangely enough, it does license gerund phrases in direct object / object complement pairs:

I expect his being at the party to cause trouble.

Another verb might license gerund / present participle pairings:

I anticipate his being at the party causing trouble.

To infinitive or not to infinitive -- that ain't the question. Posed in that way, it isn't complete enough to answer.

There's nothing wrong with using a modal verb in a subordinate clause that acts as the direct object of the verb to expect when the verb has that object as its only argument. This answer is completely independent of the fact that the verb to expect does not license present participles as object complements, even though it does license infinitives in that role. Neither of these answers breaks the standard of the other. They are two separate situations with two separate resolutions.